Tower Defense or TD is a genre of strategy computer game. The goal is to try to stop enemies - called creeps in some games - from crossing the map by building towers which shoot at them as they pass. Enemies and towers usually have varied abilities and costs. When an enemy is defeated the player earns money or points, which are used to buy or upgrade towers.
The choice and positioning of the towers is the essential strategy of the game. The Creeps usually run through a "maze", which allows the player to strategically place towers, but there are also known versions of the game which have straight lines, called line TD. The genre is highly addictive.