Orcs Must Die! is a tower-defense game where you, as the War Mage, need to protect the world from the hordes of evil creatures such as Orcs and Ogres. To complete your mission, you need to keep enemies out from magical Rifts that must not be destroyed. Every level features from four to twelve waves; each Orc that reaches your Rift damages it a little. Once the Rift Score is 0, you need to start the level over.  
The title uses a third-person perspective that makes it look like a classic action game. To protect the Rift, you can not only use different traps and spells but also fight Orcs yourself. If you're killed, you'll respawn near your Rift for a certain score.
Each trap or spell costs the in-game currency you earn by defeating enemies. At the beginning of the level, you choose up to ten items for the battle. As a rule, all traps only work once and then need some time to recharge. Every item is upgradable; on higher levels, you can also get some boosts that will affect that level only.
The game is followed by Orcs Must Die! 2 that features a co-op mode, and Orcs Must Die! Unchained, a free-to-play follow-up.