Tower defense is a sub-genre of the real-time strategy genre. 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 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. 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.