Tower defense is a subgenre of real-time strategy video games, where the goal is to stop the enemies from reaching a specific point on the map by building a variety of different 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.