Good art and audio, good variety of monsters. Overall very solid entry that I'm giving higher ratings than most.
I initially was confused at the end of the loop, not realizing that you have to kill all the monsters to progress, and I wondered if it was some kind of bug that I couldn't interact with the castle and there was just a giant cliff. I reset multiple times before realizing I had to kill all the monsters.
(Also, a very minor nitpick: the two shades of red used in the healthbar are somewhat hard to distinguish at a glance, so I was forced to look at the numbers to tell how much health I had left.)