Me and a friend tried some more endless levels. I can't remember the name of the one exactly but there is one that is 3 straight hallways with minecarts going through. It's only possible to block 2 of the doors really and there is virtually no where to put traps except on the floor. Can't say we really enjoyed that one too much.
Just started playing it solo and already it seems more challenging than the original. Though I can see how just having another person can trivialize most of the challenge.
I'm not quite sure how I feel about buying gear/upgrades straight out of the spellbook just yet. After finishing the storymode it wont matter but at this point I'm just hording skulls, unsure of what to buy. It is good that you can refund skulls spent though.
There is no penalty to refunding them, so no need to hoard them or anything. I think I respecced almost every single level on normal difficulty. Could never find a setup I liked since different traps were useful on different levels.
I'm not entirely sure how I feel about it either. On one hand I love the freedom of it and I love that there is a reason to just keep grinding various levels for skulls to upgrade stuff. You need hundreds of them to fully max out everything. But on the other hand I feel like it makes your first play through super awkward.
One of the things I liked about the first game is that each level there was a good chance the new trap you were given was going to be useful on the next level and you could plan accordingly. With the freedom of this game, though, that really isn't the case. You can buy a trap and have it turn out to be absolutely useless on the next level.... Given the free skull refund it's not a big deal but I can't say I am a huge fan of that aspect.