@Ryumaru: Thank you for the edit and your comments. It definiately gives some ideas.
@Probo: Thanks! What I forgot to mention is that I'm trying to follow most of the graphical limitations of the TI-99 home computer. The palette is given there and sadly it does not contain such nice desaturated brownish greens. The greens in it are a bit too vivid in my opininion for this kind of tiles. Nevertheless I'll see if I can come up with something with such "inverted" method.
@Mr. Farenheit: Might be. I have drawn some perspective lines first, but giving and taking some pixels here and there could have ruined the perspective.
@Mathias: It's supposed to be a pit, but since you mentioned it, I cannot unsee it being an elevation.
I'm browsing Spectrum art right now for some guidance how to work with similar limitations. Strangely the nice colorful art like the edit of Ryumaru is rarely used. Most title screens do the it colored outline/light spots on black way.
Edit:
So here's a new one based on the edit of Ryumaru. Changed the view a bit to clarify pit/elevation.
The palette has a lot of yellows/browns, thus a sepia-like shading would also be possible:
I've also tried a totally different approach:
Update2:
Well, I'm not too proud of it, but I took the easy way: