Tower Defense (TD for short) is the name of a genre of computer strategy games. The task of the player in games of this genre is to deal with the invading enemies, called in some games "creeps" (from the English creep, crawling creature), before they cross the map, by building towers that attack them when they pass close. Opponents and towers usually differ in characteristics and price. When enemies are defeated, the player earns money or points, which are used to buy or upgrade towers.