Tower defense#Gameplay

The tower defense genre can trace its lineage back to the golden age of arcade video games in the 1980s. The object of the arcade game Space Invaders, released in 1978, was to defend the player's territory (represented by the bottom of the screen) against waves of incoming enemies. The game featured shields that could be used to strategically obstruct enemy attacks on the player and assist the player in defending their territory, though not to expressly protect the territory. The 1980 game Missile Command changed that by giving shields a more strategic role. In the game, players could obstruct incoming missiles, and there were multiple attack paths in each attack wave.The Creation of Missile Command and the haunting of its creator Dave Theurer. Alex Rubens. August 15, 2013. {{cite web |url=http://www.polygon.com/features/2013/8/15/4528228/missile-command-dave-theurer |title=The creation of Missile Command and the haunting of its creator, Dave Theurer | Polygon |website=Polygon |date=August 15, 2013 |access-date=2014-06-12 |url-status=live |archive-url=https://web.archive.org/web/20140607174545/http://www.polygon.com/features/2013/8/15/4528228/missile-command-dave-theurer |archive-date=June 7, 2014 |df=mdy-all }} Additionally, in Missile Command, the sole target of the attackers is the base, not a specific player character.

While later arcade games like Defender (1981) and Choplifter (1982) lacked the strategy element of Missile Command, they began a trend of games that shifted the primary objective to defending non-player items. In these games, defending non-players from waves of attackers is key to progressing. Parker Brothers' 1982 title Star Wars: The Empire Strikes Back for the Atari 2600 was one of the first tie-ins to popularize the base defense style. The concept of waves of enemies attacking the base in single file (in this case AT-ATs) proved a formula that was subsequently copied by many games as the shift from arcade to PC gaming began. Players were now able to choose from different methods of obstructing attackers' progress.{{cite journal |last= Ellison |first= Harlan |author-link= Harlan Ellison|date= September 1982 |title= Rolling That Ole Debbil Stone |journal= Video Review|publisher= IPC Business Press|volume= 3|issue= 6|issn= 0261-3263}} (reprinted in The Comics Journal. No.85. Pg.108. October 1983. {{ISSN|0194-7869}}; later reprints in Ellison collections Sleepless Nights in the Procrustean Bed (1984) and An Edge in My Voice (1985)). Sorcerer's Apprentice for the Atari 2600 featured Mickey Mouse and was first published in 1983.

File:Nintendo Game&Watch - Green House.jpg

Nintendo's popular 1980s Game & Watch handheld games featured many popular precursors. With their fixed sprite cells with binary states, games with waves of attackers following fixed paths were able to make use of the technical limitations of the platform yet proved simple and enjoyable to casual gamers. Vermin (1980), one of the first, tasked players with defending the garden (a theme followed by many later games) from relentless horde of moles. The following years saw a flood of similar titles, including Manhole (1981), Parachute (1981), and Popeye (1981). 1982 saw multiple titles with the primary object of protecting buildings from burning: Fire Attack, Oil Panic and Mickey & Donald. The later titles utilized multiple articulating screens to increase the difficulty for players. With two screens these games introduced basic resource management (e.g. oil and water), forcing players to multitask. Green House (1982) was another popular two screen game in which players use clouds of pesticide spray to protect flowers from waves of attacking insects. Despite the early rush of archetypal titles, ultimately there was a general decline in fixed-cell games, due to their technical limitations, simplistic gameplay, and the rise of personal computers and handhelds the Game Boy; correspondingly, this genre also declined. A rare exception was Safebuster (1988 multi-screen) in which the player protects a safe from a thief trying to blow it up.

By the mid-1980s, the strategy elements began to further evolve. Early PC gaming examples include the 1984 Commodore 64 titles Gandalf the Sorcerer, a shooter with tower defense elements, and Imagine Software's 1984 release Pedro. Pedro, a garden defense game, introduced new gameplay elements, including different enemy types as well as the ability to place fixed obstructions, and to build and repair the player's territory.{{cite web |url=http://www.worldofspectrum.org/infoseekid.cgi?id=0003639 |title=Pedro |access-date=2008-12-19 |last=van der Heide |first=Martijn |work=World of Spectrum |url-status=live |archive-url=https://web.archive.org/web/20110606162255/http://www.worldofspectrum.org/infoseekid.cgi?id=0003639 |archive-date=June 6, 2011 |df=mdy-all }}

Rampart, released in 1990, is generally considered to have established the prototypical tower defense.{{cite web |url=http://palgn.com.au/article.php?id=11898 |website=PALGN |title=Tower Defense: Bringing the genre back |first=Luke |last=Mitchell |date=June 22, 2008 |access-date=December 24, 2008 |url-status=dead |archive-url=https://web.archive.org/web/20140203062250/http://palgn.com.au/11898/tower-defense-bringing-the-genre-back/ |archive-date=February 3, 2014 |df=mdy-all }} Rampart introduced player-placed defenses and has distinct phases of build, defend and repair. These are now staple gameplay elements of many games in the genre. It was also one of the first multiplayer video games of its kind.{{cite book|author=Matt Fox|title=The Video Games Guide: 1,000+ Arcade, Console and Computer Games, 1962-2012, 2d ed.|url=https://books.google.com/books?id=LVc1QNGo_g0C&pg=PA90|date=3 January 2013|publisher=McFarland|isbn=978-1-4766-0067-3|pages=90–}}

While Rampart was popular, similar games were rarely seen until the widespread adoption of the computer mouse on the PC. The DOS title Ambush at Sorinor (1993) was a rare exception from this era. Tower defense gameplay also made an appearance on consoles with several minigames in the Final Fantasy series, including a tower-defense minigame in Final Fantasy VI (1994){{cite web|url=http://ign.com/lists/top-100-rpgs/2|title=Top 100 RPGs of All Time - IGN.com|website=ign.com|access-date=May 7, 2018|url-status=live|archive-url=https://web.archive.org/web/20180304070128/http://www.ign.com/lists/top-100-rpgs/2|archive-date=March 4, 2018|df=mdy-all}} and the Fort Condor minigame in Final Fantasy VII (1997), which was also one of the first to feature 3D graphics. Attack of the Mutant Penguins for the Atari Jaguar and MS-DOS was released in 1995. Dungeon Keeper (1997) had players defend the Dungeon Heart, a gigantic gem at the centre of your dungeon, which, if destroyed, would cause the player to lose the game.{{cite web|last1=Meer|first1=Alec|title=Why Dungeon Keeper has never been beaten|url=http://www.pcgamer.com/why-dungeon-keeper-has-never-been-beaten/|website=PC Gamer|access-date=16 May 2016|date=7 August 2010}} Fortress was released for the Game Boy Advance in 2001.

As real-time strategy games gained popularity in PC gaming, the easy proliferation of user-created scenarios on Battle.net expanded the genre rapidly. Custom maps for StarCraft: Brood War such as Turret Defense (May 2000)https://scmscx.com/map/Dy5t2jBR and Sunken Defense (November 2001)https://scmscx.com/map/Tp9R9R2f were early examples of the genre that retained popularity for decades. Both were released early in the game's lifecycle and had various versions evolve in parallel over time due to the open-source nature of maps.

Warcraft III (2002), another real-time strategy game capable of such scenarios, added role-playing elements to them. The Frozen Throne expansion (2003) included a secret tower-defense scenario in one of the official campaigns. Custom maps included Element TD and Gem Tower Defense, from February 2006, which were initially created in Warcraft III World Editor.

Between 2007 and 2008, the genre became a phenomenon, due in part to the popularity of the tower defense mode in real time strategy games, but mainly due to the rise of Adobe Flash independent developers as well as the emergence of major smartphone app stores from Apple and Google. The first stand-alone browser games emerged in 2007.{{cite web |first=Michael |last=Walbridge |url=http://www.gamasutra.com/php-bin/news_index.php?story=18863 |publisher=GamaSutra |title=Analysis: Defense of the Ancients – An Underground Revolution |date=June 12, 2008 |archive-url=https://web.archive.org/web/20130521133155/http://www.gamasutra.com/php-bin/news_index.php?story=18863 |archive-date=May 21, 2013 }} Among them were the extremely popular titles Flash Element Tower Defense released in January,{{cite web |url=http://jayisgames.com/archives/2007/01/flash_element_td.php |publisher=Jayisgames |title=Flash Element TD |author=Jay |date=January 11, 2007 |archive-url=https://web.archive.org/web/20130828234150/http://jayisgames.com/archives/2007/01/flash_element_td.php |archive-date=August 28, 2013 }} Desktop Tower Defense released in March{{cite web |title=Jouer plus pour travailler moins Jeux vidéo. Sélection de petites douceurs en ligne, dangereusement addictives. |first=Erwan |last=Cario |date=November 10, 2007 |url=http://www.liberation.fr/actualite/ecrans/290440.FR.php |publisher=Libération |archive-url=https://web.archive.org/web/20080726085126/http://www.liberation.fr/actualite/ecrans/290440.FR.php |archive-date=July 26, 2008 |url-status=dead |df=mdy-all }}{{cite web |url=http://www.gamasutra.com/php-bin/news_index.php?story=21102 |title=Interview: Flash Tower Defense Creators On VC Deal, Social Gaming Site |first=Chris |last=Remo |publisher=Gamasutra |date=November 18, 2008 |archive-url=https://web.archive.org/web/20121015090411/http://www.gamasutra.com/php-bin/news_index.php?story=21102#.UiIErD_3O5I |archive-date=October 15, 2012 }} and Antbuster released in May.{{cite web |title=Antbuster: Game Nacional Online|first=David |last=Lemes |date=May 22, 2007 |url=http://gamereporter.uol.com.br/antbuster-game-nacional-online/ |publisher=Game Reporter |archive-url=https://web.archive.org/web/20150915061403/http://gamereporter.uol.com.br/antbuster-game-nacional-online/ |archive-date=September 15, 2015}}{{cite web |title=Antbuster, jogo brasileiro é premiado em disputa internacional - Papo de Homem|first=Guilherme |last=Valadares |date=June 5, 2007 |url=https://papodehomem.com.br/antbuster-jogo-brasileiro-premiado-em-disputa-internacional/ |publisher=Papo de Homem |archive-url=https://web.archive.org/web/20071231004154/https://papodehomem.com.br/antbuster-jogo-brasileiro-premiado-em-disputa-internacional/ |archive-date=December 31, 2007}} Desktop Tower Defense earned an Independent Games Festival award,{{cite web |url=http://www.gamasutra.com/php-bin/news_index.php?story=17541 |title=2008 IGF Awards Topped By Crayon Physics Deluxe |date=February 21, 2008 |publisher=Gamasutra |archive-url=https://web.archive.org/web/20130520064240/http://www.gamasutra.com/php-bin/news_index.php?story=17541 |archive-date=May 20, 2013}} and its success led to a version created for the mobile phone by a different developer.{{cite web |url=http://uk.wireless.ign.com/articles/839/839846p1.html |title=Tower Defense Review |last=Roush |first=George |publisher=IGN |date=December 5, 2007 |url-status=dead |archive-url=https://web.archive.org/web/20120330100435/http://uk.wireless.ign.com/articles/839/839846p1.html |archive-date=March 30, 2012 }} Another significant Flash title released in 2008 was GemCraft.{{cite web |url=https://www.gamedeveloper.com/game-platforms/best-of-indie-games-rose-camellia-ziczac-nameless |title=Best Of Indie Games: Rose, Camellia, Ziczac & Nameless |last=W. |first=Tim |publisher=Gamasutra |date=June 13, 2008 |url-status=live |archive-url=https://web.archive.org/web/20121023082025/http://www.gamasutra.com/php-bin/news_index.php?story=19009#.UiIHMz_3O5I |archive-date=October 23, 2012 }} Handheld game console were not ignored in the boom and titles included Lock's Quest and Ninjatown released in September and October respectively.{{cite web |url=http://www.ign.com/articles/2008/09/22/ninjatown-multiplayer-hands-on |title=Ninjatown Multiplayer Hands-on |publisher=IGN |first=Daemon |last=Hatfield |date=September 22, 2008 |archive-date=October 21, 2012 |archive-url=https://web.archive.org/web/20121021194319/http://www.ign.com/articles/2008/09/22/ninjatown-multiplayer-hands-on }} Bloons Tower Defense was first published in 2007, one of many in a series of balloon themed multi-platform games released.

The genre's success also led to new releases on PC and video game consoles. Popular 2008 titles included PixelJunk Monsters released in January, Defense Grid: The Awakening{{cite web |url=http://www.joystiq.com/2008/08/30/pax-2008-hands-on-defense-grid-the-awakening/ |title=PAX 2008 hands-on: Defense Grid: The Awakening |first=Kevin |last=Kelly |date=August 30, 2008 |work=Joystiq |publisher=Weblogs |archive-url=https://web.archive.org/web/20121028044430/http://www.joystiq.com/2008/08/30/pax-2008-hands-on-defense-grid-the-awakening/ |archive-date=October 28, 2012}} and Savage Moon in December. Also released in 2008 were geoDefense Swarm, geoDefense, GemCraft, Fieldrunners, Harvest: Massive Encounter and Crystal Defenders. GauntNet was released in April 2009.{{cite web|url=http://gauntnet.com/about.php|title=About GauntNet|access-date=September 15, 2018|archive-date=March 6, 2009|archive-url=https://web.archive.org/web/20090306001352/http://www.gauntnet.com/about.php|url-status=dead}} Plants vs. Zombies released in May 2009 was another highly popular tower defense which became a successful series on mobile devices.{{cite web |last=Ambrozy |first=Brian |url=http://icrontic.com/article/plants-vs-zombies-nominated-for-pc-game-of-the-year |title=Plants vs. Zombies nominated for PC Game of the Year |work=Icrontic |date=November 20, 2009 |archive-url=https://web.archive.org/web/20120813195707/http://icrontic.com/article/plants-vs-zombies-nominated-for-pc-game-of-the-year |archive-date=August 13, 2012}} Also released that year were Sentinel, TowerMadness, Babel Rising, Creeper World, Sol Survivor, Comet Crash, Final Fantasy Crystal Chronicles: My Life as a Darklord, South Park Let's Go Tower Defense Play!, Starship Patrol and Trenches.

With the arrival of Apple's App Store tower defense developers adapted quickly to the touchscreen interface and the titles were among the most downloaded{{fact|date=September 2024}}, many of them ported directly from Flash.{{fact|date=September 2024}} Kingdom Rush, first released in 2011, sold more than seventeen million copies both on App store and Play store.

File:Defenders of Ardania 2.jpeg]]

By the end of the boom, most tower defense games were still using side scrolling, isometric, or top-down perspective graphics. Iron Grip: Warlord, released in November, 2008 unsuccessfully pioneered the first person perspective shooter with the genre.{{cite web|url=http://www.ign.com/articles/2009/04/02/iron-grip-warlord-review|title=Iron Grip: Warlord Review|first=Jeff|last=Haynes|date=April 2, 2009|website=ign.com|access-date=May 7, 2018|url-status=live|archive-url=https://web.archive.org/web/20180112001238/http://www.ign.com/articles/2009/04/02/iron-grip-warlord-review|archive-date=January 12, 2018|df=mdy-all}} The awkward combination of experimental tower defense mechanics with 3D graphics was not well received, but later titles refined its execution paving the way for a popular new breed of games. Dungeon Defenders, released in October 2010, was one of the first tower defense games to bring the genre to the third person perspective. It sold over 250,000 copies in first two weeks of release{{cite web |url=http://www.thisisxbox.com/Arcade/dungeon-defenders-exceeds-more-than-a-quarter-of-a-million-in-sales/ |title=Dungeon Defenders Exceeds More than a Quarter of a Million in Sales|work=Internal News |publisher=ThisIsxBox.com |first=Jason |last=Andrews |access-date=9 September 2013 |archive-url=https://web.archive.org/web/20120103175813/http://www.thisisxbox.com/Arcade/dungeon-defenders-exceeds-more-than-a-quarter-of-a-million-in-sales/|archive-date=January 3, 2012 }} and over 600,000 copies by the end of 2011.{{cite web |last=Sliwinski |first=Alexander |url=http://www.joystiq.com/2011/12/22/dungeon-defenders-picks-up-gold-from-600k-sales/ |title=Dungeon Defenders picks up gold from 600K sales |publisher=Joystiq |date=2011-12-22 |access-date=2013-08-17 |url-status=live |archive-url=https://web.archive.org/web/20130330025129/http://www.joystiq.com/2011/12/22/dungeon-defenders-picks-up-gold-from-600k-sales/ |archive-date=March 30, 2013 |df=mdy-all }} 2010 saw the release of SteamWorld Tower Defense, Protect Me Knight, The Tales of Bearsworth Manor, Revenge of the Titans, Arrow of Laputa, Toy Soldiers and Robocalypse: Beaver Defense.

The 2011 title Sanctum, and its 2013 sequel popularized the first person shooter hybrid that was pioneered by these earlier games.{{cite web|url=http://www.ign.com/articles/2013/05/04/sanctum-2-merges-first-person-shooting-and-tower-defense-intriguing|title=Sanctum 2 Merges First-Person Shooting and Tower Defense. Intriguing...|first=Nathan|last=Meunier|date=May 4, 2013|website=ign.com|access-date=May 7, 2018|url-status=live|archive-url=https://web.archive.org/web/20180111182324/http://www.ign.com/articles/2013/05/04/sanctum-2-merges-first-person-shooting-and-tower-defense-intriguing|archive-date=January 11, 2018|df=mdy-all}} Orcs Must Die! also integrated the FPS genre into a fully 3D environment and went to have several sequels. Anomaly: Warzone Earth released in 2011 introduced a variation of gameplay which has been described as "reverse tower defense",{{cite web |url=http://toucharcade.com/2011/03/22/gdc-2011-anomaly-warzone-earth-tower-defense-in-reverse/ |title=GDC 2011: Upcoming 'Anomaly Warzone Earth' – Tower Defense in Reverse |publisher=Touch Arcade |first=Blake |last=Patterson |date=March 22, 2011 |access-date=March 30, 2011 |url-status=live |archive-url=https://web.archive.org/web/20110326023646/http://toucharcade.com/2011/03/22/gdc-2011-anomaly-warzone-earth-tower-defense-in-reverse/ |archive-date=March 26, 2011 |df=mdy-all }} "tower attack",{{cite web |url=http://blog.games.yahoo.com/blog/260-tower-defense-gets-turned-on-its-head-in-anomaly-warzone-earth |title=Plugged In - Tower defense gets turned on its head in Anomaly: Warzone Earth |publisher=Yahoo! Games |first=Gordon |last=Cameron |date=December 14, 2010 |access-date=April 9, 2011 |url-status=dead |archive-url=https://web.archive.org/web/20110125213943/http://blog.games.yahoo.com/blog/260-tower-defense-gets-turned-on-its-head-in-anomaly-warzone-earth |archive-date=January 25, 2011 |df=mdy-all }} and "tower offense".{{cite web |url=https://www.gameinformer.com/games/anomaly_warzone_earth/b/pc/archive/2011/04/08/tower-offense-delivers-unusual-thrills.aspx |title=Anomaly: Warzone Earth - Tower Offense Delivers Unusual Thrills |publisher=Game Informer |first=Adam |last=Biessener |date=April 8, 2011 |access-date=April 10, 2011 |url-status=live |archive-url=https://web.archive.org/web/20110415053358/http://www.gameinformer.com/games/anomaly_warzone_earth/b/pc/archive/2011/04/08/tower-offense-delivers-unusual-thrills.aspx |archive-date=April 15, 2011 |df=mdy-all }} In the game, the player must attack the enemy bases protected by numerous defenses. Sequels and other games have since experimented further with both styles of tower defense.{{cite news|url=http://www.pcgamer.com/anomaly-defenders-announced-reverses-tower-offense-gameplay/|title=Anomaly Defenders announced, reverses 'tower offense' gameplay|website=pcgamer.com|date=April 3, 2014 |access-date=May 7, 2018|url-status=live|archive-url=https://web.archive.org/web/20170802061723/http://www.pcgamer.com/anomaly-defenders-announced-reverses-tower-offense-gameplay/|archive-date=August 2, 2017|df=mdy-all|last1=Maiberg |first1=Emanuel }} Tiny Heroes, Army of Darkness: Defense, Iron Brigade, Rock of Ages and Trenches 2 were also released in 2011.

Defender's Quest, Bad Hotel, Toy Defense, Strikefleet Omega, Unstoppable Gorg, Defenders of Ardania, Orcs Must Die! 2, Fieldrunners 2, Dillon's Rolling Western, Oil Rush and Elf Defense Eng all came out in 2012. Around this period the genre matured, gaining recognition as a distinct sub-genre of strategy games and returning in numerous upgraded versions. Chain Chronicle and CastleStorm were released in 2013. Plants vs. Zombies 2 came out and Prime World: Defenders featured deck-building mechanics. 2014 saw a number of brand new titles including Space Run, Dungeon of the Endless, Island Days, Final Horizon and The Battle Cats as well the return of Age of Empires: Castle Siege, Defense Grid 2 and TowerMadness 2. Deathtrap and Krinkle Krusher were first published in 2015.

More recent titles in the genre include Rock of Ages 2: Bigger & Boulder (2017), Tower Battles (2017) and Orcs Must Die! Unchained (2017), Dillon's Dead-Heat Breakers (2018), Eden Rising: Supremacy (2018), Aegis Defenders (2018), Bloons TD 6 (2018), Arknights (2019), Taur (2020), Element TD 2 (2020), Buster's Tower Defense (2021), and Path to Nowhere (2022).

With the advent of social networking service applications, such as the Facebook Platform, tower defense has become a popular genre with titles such as Bloons TD and Plants vs. Zombies Adventures making the transition to turn-based play.{{cite news|title=Plants vs. Zombies Adventures Announced for Facebook|url=http://news.softpedia.com/news/Plants-vs-Zombies-Adventures-Announced-for-Facebook-341114.shtml|date=July 14, 2014|url-status=live|archive-url=https://web.archive.org/web/20160927172127/http://news.softpedia.com/news/Plants-vs-Zombies-Adventures-Announced-for-Facebook-341114.shtml|archive-date=September 27, 2016|df=mdy-all}} Recent releases include Star Fox Guard and McDroid which came out in 2016.

File:Tower defense (micro td).gif tower defense game for Arduboy]]

The basic gameplay elements of tower defense are:

• territories, possessions, domiciles, (collectively, the "base") that must be defended by the player or players
• waves of multiple incoming "enemies" that must be defended against
• placement of "tower" elements, such as towers, or obstructions along the path of attacking enemies

What distinguishes tower defense base defending games from other base defending games (such as Space Invaders, or other games where bases are defended) is the player's ability to strategically place, construct or summon obstructive constructions and constructive obstructions in the path of attacking enemies.

In a tower defense, the player's main character is often invincible, as their primary objective is the survival of the base rather than the player.

Some features of modern tower defense:

• Player-placed obstructions that can damage or kill enemy attackers before they destroy the base
• Ability to repair obstructions
• Ability to upgrade obstructions
• Ability to repair upgrades to obstructions
• Some sort of currency with which to purchase upgrades and repairs, this can be time, in-game currency or experience points, such as being earned by the defeat of an attacking unit.{{cite web |url=https://www.wsj.com/articles/SB117987060189311315 |title=Strategy Game Pits Players Against Desktop Invasion |last=Rutkoff |first=Aaron |publisher=Wall Street Journal |date=June 20, 2007 |access-date=August 31, 2013 |url-status=dead |archive-url=https://web.archive.org/web/20170809132150/https://www.wsj.com/articles/SB117987060189311315 |archive-date=August 9, 2017 |df=mdy-all }}
• Enemies capable of traversing multiple paths at once
• Each wave usually has a set number and types of enemies
• Unlock new maps and levels
• Tower that have the ability to move

Many modern tower defense games evolved from real-time to turn-based, cycling through distinct gameplay phases such as build, defend, repair, and celebrate.

Many games, such as Flash Element Tower Defense feature enemies that scamper through a "maze", which allows the player to strategically place "towers" for optimal effectiveness.{{cite web |url=http://www.novelconcepts.co.uk/FlashElementTD/ |title=Flash Element Tower Defense |last=Scott |first=David |access-date=April 15, 2009 |url-status=dead |archive-url=https://web.archive.org/web/20090414043003/http://www.novelconcepts.co.uk/FlashElementTD/ |archive-date=April 14, 2009 |df=mdy-all }} However, some versions of the genre force the user to create the "maze" out of their own "towers", such as Desktop Tower Defense.{{cite web |url=http://www.handdrawngames.com/DesktopTD/game.asp |title=Desktop Tower Defense |publisher=Handdrawngames.com |last=Preece |first=Paul |access-date=September 25, 2008 |url-status=dead |archive-url=http://arquivo.pt/wayback/20090710075045/http%3A//www.handdrawngames.com/DesktopTD/game.asp |archive-date=July 10, 2009 |df=mdy-all }} Some versions of the genre are a hybrid of these two types, with preset paths that can be modified to some extent by tower placement, or towers that can be modified by path placement, or modifications that can be placed by tower paths.

Often an essential strategy is "mazing", the tactic of creating a long, winding path of towers (or "maze") to lengthen the distance the enemies must traverse to get past the defense. Sometimes "juggling" is possible by alternating between barricading an exit on one side and then the other side to cause the enemies to path back and forth until they are defeated. Some games also allow players to modify the attack strategy used by towers to be able to defend for an even more tantalizingly reasonable price.{{ cite web |url=http://www.creeptd.com/en/manual |title=CreepTD manual |access-date=February 13, 2012 |url-status=live |archive-url=https://web.archive.org/web/20120228002835/http://www.creeptd.com/en/manual |archive-date=February 28, 2012 |df=mdy-all }}

The degree of the player's control (or lack thereof) in such games also varies, from games where the player controls a unit within the game world, to games where the player has no direct control over units at all, or even no control over the game whatsoever.

A common theme in tower defense games to have occasional "air" units which ignore the layout of the board (i.e. maze paths or obstructions) and travel directly to the end destination, or enemies which prioritize different targets than their main destination.

Some tower defense games or custom maps also require the player to not only defend their own board but send out enemies to attack their opponents' game boards (or opponent-controlled areas of a common game board) in return. Such games are also known as tower wars game boards.

On June 3, 2008, COM2US Corporation was awarded the trademark for the term "Tower Defense", filed on June 13, 2007 – serial number 3442002. The corporation started enforcing the trademark: in early 2010, developers of games on Apple's App Store received messages requiring name changes for their games, citing trademark violation.{{cite news|url=http://www.pocketgamer.biz/news/17997/com2us-guides-developers-not-to-use-its-trademark-tower-defense/|title=Com2uS "guides" developers not to use its trademark Tower Defense|last=Jordan|first=Jon|date=January 25, 2010|work=PocketGamer.biz |archive-url=https://web.archive.org/web/20130530143312/http://www.pocketgamer.biz/r/PG.Biz/Com2uS+news/news.asp?c=17997 |archive-date=May 30, 2013}}{{cite web|last1=Slashandburn/Mgate Labs|title="Tower Defense" is now up for CP Infringement|url=http://forums.toucharcade.com/showthread.php?t=42234|website=Touch Arcade|access-date=May 8, 2015|date=January 21, 2010|url-status=live|archive-url=https://web.archive.org/web/20150518081915/http://forums.toucharcade.com/showthread.php?t=42234|archive-date=May 18, 2015|df=mdy-all}} Adding the phrase "Tower Defense" (in capital letters) to the description of an app submission to iTunesConnect and the app store automatically triggers a warning that the submission is likely to be rejected for use of the term; however, writing the phrase in lower case is still acceptable as "tower defense" is a valid description of a game style.{{fact|date=September 2024}}

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Category:Video game genres