Okay, played the game for 4+ hours, got a total of 6 characters and cleared the first multiplayer dungeon with a friend 3 or 4 times (it's really short).
It's definitely better than the first game, and the only "tower defense" element so far was protecting an NPC while trying to stop waves of enemies and destroy gates on the other side of the map.
Speaking of maps, I blame myself, but I expected more of an "open world" than quite big maps with randomly scattered monsters. Areas feel really empty and there's not really that much to do. I've only cleared the first 2 though, so it might get better later on.
That said, despite the above, I'm enjoying Heroes 2 way more than 1. It feels much more like a cross between a musou and an action RPG than between a musou and a tower defense game.
As for the multiplayer dungeons, the first is a series of 3 rooms (2?) and a boss. Aside from the boss fight, which was made with multiple people in mind, it's really straightforward and fun chaos with summoning monsters, becoming monsters and everyone using their special attacks.
Can't wait to play more over the weekend.
You can check out the first 4 hours of the game
here, once YT finishes processing my stream.