Orcs Must Die!
is one of those rare games where the title tells you everything you need to know about gameplay. You play as a rookie War-Mage tasked with keeping orcs from escaping through Rifts (portals) after your master has an unfortunate mishap with kobold blood and the pavement.
The core idea behind Orcs Must Die! isn't much different than other Tower Defense games. You're handed an assortment of damage-dealing traps to set up along a set path leading to the Rift. Where Orcs Must Die! differs is how you place traps; between waves, you run through each maze, figuring out the best places to drop traps. Sometimes, it is as easy as finding a choke point, though most of the time, you're faced with multiple hallways to defend, forcing you to really think about how you're going to shepherd the incoming hordes through the level.
Another difference is you can take a more hands-on approach to Rift defense. Rather than setting traps and hoping for the best, you can arm yourself with a number of spells and weapons to help hold back the crush of enemies. You start with a crossbow and blade staff, both of which are great equalizers, but eventually gain access to items like the whirlwind belt, which not only lets you hold enemies in place, but also sends a gust of wind into enemies, knocking them into traps. Eventually, you add Weavers to your arsenal - sorceresses who can add extra protection and spells.
Since you're only allowed to bring a set number of traps and abilities into a level, situations become more puzzle-like. Orcs Must Die! isn't something you can easily master; there are simply too many solutions to each predicament. Yet, it is something you can sit down with and play for hours, even if you find yourself constantly overrun by certain enemy configurations.
The one downside is longevity; once you've cracked a level and earned a five-Skull rating, there isn't much incentive to return, unless you're aiming for the top spot on the leaderboard. You'll also unlock Nightmare difficulty, which is a cool bonus for hardcore Tower Defense nuts only.