Wow, people weren't kidding when they said "old school" lol!
Awesome game, lots of fun. The knockback makes it so that if you have perfect timing, you can infinitely repel enemies. It's decently difficult to have perfect timing 100% of the time, though, so it's a good challenge with a higher skill ceiling. That the player can fall through the floor is also a great additional challenge. Pair that with the enemies' knockback, and the stakes get pretty high.
Good level design, too.