Here's my first whack at areas C & D. I'm not sure what to put here but the space seemed to lend itself to the idea of a jail cell.
That and I wanted to see if I could make walls look 'right' and add in a nest of hay for a bed.
Same area though this time I played with the idea of a series of stone steps over pools of green, glowing slime.
The rock-steps are...'eh' but I'm not sure how to anchor them to the ground yet - to make them feel like they're flat-ish paving stones.
The green pools are just an experiment with layers of different textures and a glow effect.
Lightning the tops and darken the sides if you want them to look more flat, or do sharper shadows on the edges. I'd say they would look more anchored if they interacted with the slime more, ripples around them or some sense they were affecting its growth or flow pattern.
Added some slime growth on the stones and some ripples around them within the pools. Also added some shading to the stones to try and flatten them a bit.