I really like the concept. At first I thought this would just be a normal tower defense game. However I think you managed to really make it feel like you are playing as all the small enemies in a normal castle level. The art and music is also great! The hero AI feels very exploitable though. Skeletons can do a lot of damage if they run back and forth. Bats can literally just kill the hero if they continuesly make the hero climb a ladder and always attack them on the ladder, where the hero can't attack.
Overall this was a fun game though, good job!