Wonderful concept. Really fun idea. As usual an exceptionally high quality entry. I think the UI is perfect - I can very quickly use the equipped items. I liked exploring the outside of the tower - completely unnecessary and absolutely wonderful. Touches like that make me really like your games.
I'm kind of a sucker for upgrading items so I'm glad I got to do that.
The combat didn't feel great to me, I basically felt like I was spamming attacks and constantly digging through the inventory to get to appropriate element attacks. Then I went up a level and the inventory problem went away but I still found it a bit tedious.
The core gameplay loop didn't really hold me, I wasn't sure what I had to look forward to other than more levels and more hats. Still that was kind of cool.
I haven't had a chance to play multiplayer but I suspect it will be a very different experience when I do.