I'm fine if it's more difficult without a partner, as long as the later levels are still beatable. I'm hoping it doesn't suffer from the problem/feature that started to creep into Orcs Must Die 2 where some levels are designed specifically to take advantage of having a second character (making you guard 4-6 separate paths rather than 2-3).

I've only played the demo on 360 and, while single player seems possible, it's definitely geared toward having a buddy with you, or at least a frothing monkey found in matchmaking.

On xbox you can easily solo pre-martian bear dlc while using and engineer class Trench. I wasn't able to gold them all but you will be able to beat it at least. However the dlc is almost required to have a co-op as the difficulty ramped up fast. That was the xbox I'm assuming it's close for the pc version.