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Tower defense (TD) is a subgenre of strategy video game where the goal is to defend a player's territories or possessions by obstructing the enemy attackers, usually achieved by placing defensive structures on or along their path of attack.^[1] This typically means building a variety of different structures that serve to automatically block, impede, attack or destroy enemies. Tower defense is seen as a subgenre of real-time strategy video games, due to its real-time origins,^[2][3] though many modern tower defense games include aspects of turn-based strategy. Strategic choice and positioning of defensive elements is an essential strategy of the genre.

Ryan Clements of IGN attributes the popularity of such games to to psychology and human vulnerability.^[4] Tower Defense, according to Clements, "plays off of our need for security, ownership, and a desire to protect the people closest to us" arising from a need to create intrinsic value through "ownership over things", "personal space" and to "repel our fears and insecurities".^[4]

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 which could be used to strategically, to obstruct enemy attacks on the player and assist the player to defend their territory, though not specifically to protect the territory. The 1980 game Missile Command changed that by introducing a strategy element. In the game, players could obstruct incoming missiles, and there were multiple attack paths in each attack wave.^[5] Missile Command was also the first of its kind to make use of a pointing device, a trackball, enabling players to use a crosshair. The innovation was ahead of its time and anticipated the genre's later boom, which was paved by the wide adoption of the computer mouse. Additionally, in Missile Command, the sole target of the attackers is the base, not a specific player character. For these reasons, some regard it as the first true game in the genre.^[5]

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.^[6]

Nintendo's popular 1980s Game & Watch hand held 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, had 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 more notably: Manhole (1981) players could build temporary obstructions to save pedestrians; Parachute (1981) players used a boat to save parachutists from hungry sharks and in Popeye (1981), a game based on the Popeye franchise, protect Olive Oyl from an endless stream of objects thrown at her. 1982 saw multiple titles with the primary object of protecting buildings from burning: Fire Attack, Oil Panic and Mickey & Donald. The later titles utilised 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 multi-task. 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 the technical limitations, simplistic gameplay, and the rise of personal computers and handhelds the Game Boy saw a general decline in fixed cell games and likewise the genre. 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.^[7]

Rampart, released in 1990 is generally considered to have established the prototypical tower defense.^[8] Rampart introduced player placed defenses that automatically attack incoming enemies. In addition, it 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.^[9]

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.^[9] 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)^[10] and the Fort Condor minigame in Final Fantasy VII (1997), which was also one of the first to feature 3D graphics. As Real Time Strategy games gained popularity in PC gaming, many introduced tower defense modes in their gameplay, particularly in multiplayer modes. The 2006 mods Element Tower Defense (Element TD) and Gem Tower Defense released in February for the popular RTS title Warcraft III: Reign of Chaos almost single-handedly rekindled the genre.^[8] These titles would also bring role-playing elements to the genre for the first time.

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.^[11] Among them were the extremely popular titles Flash Element Tower Defense released in January^[12], Desktop Tower Defense released in March^[13][14] and Antbuster released in May^[15][16]. Desktop Tower Defense earned an Independent Games Festival award,^[17] and its success led to a version created for the mobile phone by a different developer.^[18] Another significant Flash title released in 2008 was GemCraft.^[19] Handheld game console were not ignored in the boom and titles included Lock's Quest and Ninjatown released in September and October respectively.^[20]

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, many of them ported directly from Flash. Among the more notable include Bloons TD 4 (2009) which sold more than a million copies on iOS.^[21]

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^[22] and Savage Moon in December.^[8] Plants vs. Zombies released in May 2009 was another highly popular tower defense which became a successful series on mobile devices. ^[23]

By the end of the boom, most tower defense games were still stuck in the side scrolling, isometric, or top-down perspective graphical medium. Iron Grip: Warlord, released in November, 2008 unsuccessfully pioneered the first person perspective shooter with the genre.^[24] 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^[25] and over 600,000 copies by the end of 2011.^[26] The 2011 title Sanctum, and its 2013 sequel popularised the first person shooter hybrid that was pioneered by these earlier games.^[27]

Anomaly: Warzone Earth released in 2011 introduced a variation of gameplay which has been described as "reverse tower defense",^[28] "tower attack",^[29] and "tower offense".^[30] 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.^[31]

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.^[32]

The basic gameplay elements of tower defense are:

• territories or possessions (or collectively the "base") that must be defended by the player^[1]
• the base must survive waves of multiple incoming "enemy" attacks
• placement of "Tower" elements, or obstructions along the path of attacking enemies^[1]

What differentiates tower defense from other base defending games (such as Space Invaders) is the player's ability to strategically place or construct obstructions in the path of attacking enemies.

In Tower defense, unlike the base, the player's main character is usually, but not always, invincible, as the primary object 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 destroying the base
• Ability to repair obstructions
• Ability to upgrade 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^[33]
• Enemies capable of traversing multiple paths
• Each wave usually has a set number and types of enemies

Many modern tower defense games evolved from real-time to turn based gameplay in which there is a cycle including distinct phases such as build, defend and repair. Many games, such as Flash Element Tower Defense feature enemies that run through a "maze", which allows the player to strategically place towers for optimal effectiveness.^[34] However, some versions of the genre force the user to create the maze out of their own towers, such as Desktop Tower Defense.^[35] Some versions 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. Often an essential strategy is "mazing", which is the tactic of creating a long, winding path of towers 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 reasonable price.^[36]

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.

It is a common theme in tower defense games to have air units which do not pass through the layout of the maze, but rather fly over the towers directly to the end destination.

Some tower defense games or custom maps also require the player to send out enemies to their opponents' game boards respectively their controlled areas at a common game board. Such games are also known as tower wars games.

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 is reported to have started enforcing the trademark: in early 2010, developers of games on Apple's App Store reported receiving messages requiring name changes for their games, citing trademark violation.^[37][38] 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.

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