The brilliant sequel to 2011's Orcs Must Die added more traps, more enemies, more levels, and, most importantly, a second player to help out. Orcs Must Die 2 added so many new balls to juggle that its single-player mode became crowded and confused, while co-op became the star of the show. Played alone, this tower defense-alike can be a punishing slog against hoards of enemies swarming in from all directions. With a friend, though, you've got backup and a second set of hands to fight back the darkness. Placing traps and settling in to fight a stream of bad guys is so much more rewarding (and tactically sound) when you know that your friend has got the other door covered.