This is a game about killing orcs. The concept is simple enough, but the execution is high quality.
The game is basically a 3rd person shooter in which you have to defend a magical rift in a castle from a horde of orcs and other monsters that are trying to get through it. If too many monsters make it to the rift you lose.
One man against a relentless horde of monsters may not seem fair, but you have a lot of advantages. In addition to your trusty crossbow, you'll have a large arsenal of spells, traps, and guardians to help out. Furthermore you earn skulls for completing levels, and you can then spend these skulls on further upgrading your arsenal.
At the start of each level you'll have time to plan and set up your defenses and get a feel for the layout of the area. You'll be told where the orcs are coming from, what route they are likely to take, how many waves there are, and even what monsters you can expect to come knocking down your door.
This is important because, despite the simple premise of "orcs must die," this is really a tactician's game. The first few levels you may be able to survive using just your crossbow, but as the difficulty level increases you will have to diversify your tactics or perish. You'll start facing more orcs, tougher monsters, monsters that come from multiple entrances, and even multiple rifts that must be defended simultaneously.
I will give one bit of warning that is not mentioned in the game-- if you intend to play on the default (war mage) difficulty level as I did, in order to earn the maximum of 5 skulls per level you not only need to prevent any orcs from reaching the rift, you've also got to do it within the par time. Once you start the initial wave the timer starts, and it doesn't stop even during the "break" periods you sometimes get between waves.
The game is single player only. The graphics and music are adequate, but not AAA quality (this is an indie game, after all, and I don't expect AAA quality here).
There is a storyline which is short, simple, and entertaining. Borderlands fans will recognize the voice of Axton the Commando as the War Mage.
If you like third person shooters or defense-centered gameplay you'll like Orcs Must Die!