All in all, this was a pretty fun game, but there are some serious bugs that have to be fixed with it.
For starters, in the later levels (Chapter 2), some enemies become "invincible" - they kill your monsters and your monsters don't attack them back. This seems to only happen to one monster per stage, so you can move onto the next stage by clearing all your rooms and letting that one monster walk out with one treasure chest as long as you still have one to spare. This is the biggest bug that I've found with the game. Some people have suggested that it may be triggered by using the fear spell?
Next up, another bug that people have been talking about is the bug with the upgrades. The upgrades for the final monsters (Champions? - Beholder and Dragon), as in Nest, Shield etc upgrades that cost 20 stars don't seem to work for those monsters. When they are bought in the upgrade book, the upgrades show up next to the tomb monsters. This should be fixed as well.
Another bug: The ghost monsters don't seem to regenerate health ever. Is this intended?
Also, for a lot of monsters, the "Improved leveling" upgrade (Lower right corner of each 2 page set) doesn't seem to function properly - the monsters can't seem to upgrade past one red star in most cases. I'd also like the leveling system to be explained a bit more.
Some suggestions: You should be able to SELL monster rooms rather than CLEAR the rooms, allowing you to change up room layouts without putting yourself too far behind.