Admittedly, it's been a little while since I played Orcs Must Die, but it's not a game that I would say rests heavily on its "plot". Essentially Orcs constantly come charging into your castle, and you have to stop them. With murder.
Why "must" they die? Who knows. In fact, the entire game could simply be skewed by point of view. The more you think about it, there are a lot of games out there in which we simply take it on faith that we are playing a good guy, and doing the right thing by slaughtering thousands of orcs/bandits/whatever.
However, moral crises of conscience aside, there's no arguing that Orcs Must Die was a lot of fun.
OMD is a tower defense type game, taken to the next level. When OMD first came out, there weren't many tower defense games that put you right into the action, and none that I can think of that did so with such polish and perfection of execution.
The orcs stream into your fortress in waves and (for some reason) run headlong to their goal. It's up to you to strategically place a variety of traps along their path, and often even jump into the fray yourself in order to thin the horde. It was at times frustratingly challenging, but always a good time.
Orcs Must Die 2 comes out today, and while it adds more of the same orc-killing action that the original featured, OMD2 adds co-op, which elevates it to a whole new level. I expect OMD2 to be about forty kablillion times better as a result, and I'm looking forward to digging into it with a friend later on this evening.
And then later this week, finally, the one Summer of Arcade game I've been waiting for. Deadlight. Shaping up to be a pret-tay good week.