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A prototypical tower defense game was the Atari Games release Rampart in 1990. By the early 2000s, maps for StarCraft, Age of Empires II, and Warcraft III were following in Rampart's footsteps. The Fort Condor strategy minigame in Final Fantasy VII may also have inspired the tower defense genre as it contains several elements that are now genre mainstays.
A screenshot of Defenders of Ardania showing the genre's characteristic towers, as well as units and a castle that serves as an end point
Tower defense games are characterised by the positioning of static units by the player to defend against mobile enemy units who are trying to get from a start point to an end point. There is a set number of enemy units (or 'damage' the player can take from units reaching the end point) who can reach the end point before the level is lost. Some games use a static route that the enemy units follow around which the player places their towers, while others favour a free-form environment that allows the user to define the path the enemy units take. Some games use a mixture of both. Most games allow the upgrading of the player's towers.
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.
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 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.