I get that you chose beast and gremlin because they are both resistant to the same damage type, but the way you described the level seems like there should at least be undead here. However, since there should be at least 2 monster types (doesn't have to be, but that would be nice), and undead+fiend combo is too overused, I guess it's ok. Or you could go with undead+gremlin, since one is weak to elemental and the other is resistant to it.

Gremlins could have a new monster type that are like the thwackers, but can deal curse status with their spin attack (low chance). Carnavons should also spawn in certain pre-set areas. This would give the levels a more "cursed" feeling and more greatly link it to its Almirian lore.

There could also be a new undead monster type: The Almirian Guard (not to be confused with the Almirian Royal Guard). This new monster would function similar to the Almirian crusaders in that they are the same size and move similarly, but they would have a sword and shield instead of a lance and shield (possibly give them a FoV shaped sword, or a new sword that is newly designed just for them). The only real difference between them and Crusaders is that they would have a 3-hit sword combo for a normal attack, and their charge would be a single sword blast (exactly like the 4* avenger). They would also look exactly like the SL Almirian Crusader Armor set with the shield included. These monsters could serve as the guards to the cursed city forever protecting their home, thus fitting perfectly into the lore you created. There would be a status version for every variety of floors you created, with the difference in each's appearance being the color of the sword and the Guard's own personal color.

Payout for a single floor should be slightly less than it's tiered boss equivalent, and only 2 floors can occur in a row (make a layout that would serve as the boss floor for the themed floors, much like Briar Bone Barrage for the Concrete Jungle stages). The theme should appear in any undead stratum besides T1, and only in the stratum just before the next sub-town.

TL;DR
A new undead and gremlin monster, and change the 2 monster types to undead and gremlin. Possible to be cursed in this floor from monsters, but small chance.

P.S. Sorry it took me so long to respond to this thread. I had fully intended to post on the day you first made it, but I kinda forgot :P

What are those Gremlins up to in those floors? Usually, the Gremlins have some sort of agenda whenever they pop out in a place that isn't a Clockworks Tunnel or Arena. Like in the Jigsaw Valley where they're chopping down trees or the Deconstruction Zones where they're ripping a floor apart for spare bits and pieces. Are they using those old Almirian city fragments as some sort of staging area?