Orcs Must Die 2 continues right where 1 left off. The much-awaited multiplayer experience is here, but this review will be from a single-player standpoint.
To start with, you get to pick between the Warmage and the Sorceress. Both have their strengths and weaknesses, but the Warmage simply feels easier to play with due to his superior health, superior tar trap, and arguably superior weapon. The Sorceress is better for charming single, large targets though, and her acid trap does more damage than the Warmage's Arrow Trap.
Gameplay is more or less the same. If you played OMD1, this will all be familiar. The Elementalist girls are gone, which is a shame. This change means that you have less options to adjust to your enemies in the middle of a level. Fortunately any upgrades you purchased prior to battle can be reset at no cost. Autoballistae are now considered guardians, which means you can no longer spam them. I was sad about that as well.
Amazingly they made the Gnolls more irritating. They seem to have more HP, run faster (both probably untrue, but it feels like it), and can now slow you upon hitting you, making escape impossible. A whole pack of them, especially in Endless mode (see below), will ruin your day.
I'm rather disappointed that the campaign is so short. The first game took maybe 15-20 hours to complete, and had a good progression. OMD2 instead took me a mere 10 hours on normal difficulty. Granted, I already knew the basics of the game, but this was far shorter than I expected. DLC's extend the game's length a bit, but they contain no story segments, and thus you are basically playing for skulls (the game's peristent currency) and variety. Note that you'll have to play through Campaign again with another character if you want stuff unlocked on both characters.
Endless mode is interesting. Waves come indefinitely and keep getting tougher until you can no longer handle it, much like Survival mode in other types of games. This mode by far is the biggest time-sink, though you are rewarded with plenty of skulls to compensate.
If you like the first game and want a way to play it with friends, then OMD2 is the way to go. Overall it's an acceptable game. I still recommend the first game though.