What are tower defence games?

Tower defense is a video game genre in which players are usually tasked with defending a fixed point by building various stationary defenses. Some popular examples of the genre are the "Orcs Must Die" series, the "Bloon" series and the "Plants vs. Zombies" series.

The arcade game "Rampart" is widely thought to be the first tower defense game. Players usually face a large amount of enemies assaulting their position and have only limited resources with which to build fixed defenses such as cannons. The games are often organized in "waves" with players given a limited amount of resources or time between waves to repair damage to their defenses.