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Tower defense is a subgenre of real-time strategy video games. The goal of tower defense games is to try to stop enemies from crossing a map by building traps to slow them down and towers which shoot at them as they pass. Enemies and towers usually have varied abilities, costs, and upgrade prices. When an enemy is defeated, the player earns money or points, which are used to buy or upgrade towers, or upgrade the number of money or points that are earned, or even upgrade the rate at which they upgrade.
The choice and positioning of the personal towers is the essential strategy of the game. 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. However, some versions of the genre force the user to create the maze out of their own towers, such as Desktop Tower Defense. 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.
Eventually, independent video game developers began using Adobe Flash to make stand-alone tower defense browser games, which led to the release of Flash Element Tower Defense in January 2007 and then Desktop Tower Defense in March of the same year. Desktop Tower Defense became immensely popular and earned an Independent Games Festival award, and its success led to a version created for the mobile phone by a different developer. Several other tower defense video games achieved a level of fame, including Protector, Immortal Defense, GemCraft, and Plants vs. Zombies.
By 2008, the genre's success led to tower defense games on video game consoles such as Defense Grid: The Awakening on the Xbox 360, and PixelJunk Monsters and Savage Moon for the PlayStation 3. Tower defense games have also appeared on handheld game consoles such as Lock's Quest and Ninjatown on the Nintendo DS.
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.
USPTO trademark 
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 has been reported to start enforcing the trademark; specifically on the iOS platform as a number of small or independent developers are now receiving messages from Apple citing trademark violation.
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