Uterus in a Castle is a bit weird as he is a WATER enemy.
The background in Layer 8 is not bad, but you need something in layer 6 or 7. Also, try to vary what tiles you use in Layer 4 and make the platforms bigger as one tile high platforms look boring.
The boss was a bit easy as I had 60+ Seeker ammo and killed it in about 30 seconds.