I extremely enjoyed this game, and hope to see more of this quality in the future. I've all but given up on Newgrounds to provide anything meaningful, but this was one of the few that stood out so that I was compelled to share it with my friends.
The music was great, the length was surprisingly more than I expected for a flash game, the gameplay was addictive despite it's simplicity, and I was satisfied with the boss battle and the dungeon/castle itself.
The only things I could suggest for another installment is to make it longer and add more content. Puzzles, enemies, items, etc. Overall, great job.