Complex sentence: 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.
Split into simpler sentences: Tower defense is a subgenre of real-time strategy video games. The goal of tower defense games is to try 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.