Today’s gamer vocabulary word is “Tower Defense”.
Tower defense is a subgenre of real-time strategy video games. 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
A tower defense game is split into two phases which alternate back and forth. In the first phase, the player spends the gold they have building defenses. This might include tower that shoot different ammunition or traps meant to stop the enemies (or creeps). In the second phase, the creeps start walking, running, flying, whatever from one end of the map trying to make it to the other. Once they are all killed, the first phase starts again. The game is over when enough creeps make it across the map or the player defeats all the levels.
You can get an idea of what the gameplay in like from this video of Defense Grid: The Awakening.
There are some games that tweak this formula. In Sanctum, the player takes control of a character in phase 2 and fights the creeps directly. And then in Anomaly Warzone Earth the player plays as the creeps trying to avoid the towers.