Donut Fortress
Basically I avoided using most of the enemies used in Morton's Castle considering that's where the game introduces it's thwomps, thwimps, and drybones. Of course, I used only Koopas and the Ball n' Chain to make it so the Ball n' Chain is first introduced in this fortress... but the level is more focused on Koopas.
Also has a secret exit.
How to get secret exit (for those who are stumped):
Take a shell to the end of the first area, and hit the turn block that's above the door. This gives access to the Silver P-switch, clearing away the munchers to the next door. Next, hit the Blue P-switch and find the Blue Shell. Afterwards, hit the [?] Blocks that are at the top of the screen until you gain a key. Afterwards, just climb the [?] Block "stairs" and get the secret exit.
Want progress on 100 Rooms of Enemies: The Nightmare Edition? Go
here to see.
(rip my other userbars momentarily)