Tower defense games have a very simple layout. Usually, computer-controlled monsters called creeps move along a set path, and the player must place, or “build” towers along this path to kill the creeps.
In some games, towers are placed along a set path for creeps, while other towers can interrupt creep movement and change their path. In most tower defense games different towers have different abilities such as poisoning enemies or slowing them down. The player is awarded money for killing creeps, and this money can be used to buy more towers, or buy upgrades for a tower such as increased power or range.