This is a really good start Alecthar! Make sure you put each set of elements on different layers so they are easy to correct (if you are not already) if your pillars are on a different layer to the rest of the map then it's a snap to delete them and start again without destroying your other work.

I use photoshop and not Gimp, but many of the basic principles are the same. One huge advantage that photoshop has over Gimp is layerstyles, but if you're not getting photoshop then don't worry about them

Basic principles:

1. make sure each element is on its own layer for easy editing.
2. use layer masks to make your textures more interesting. For example, if you wanted to make the floor texture more interesting then duplicate your floor layer and then texture that with a different texture. Use a layer mask and render clouds on it to get a smooth gradation between the overlying and underlying texture. In mapping, using clouds as layer masks is a really useful trick and one you should use often if you want to get interesting textures.
3. I think the raised floors look fine. Once you pop some water in them (on it's own layer) and use the cloud / layer mask method to make your water more interesting then it should start looking really good.
4. Don't use gradients to make stairs, use the drop shadow instead, it's much faster and gives much the same results for this purpose.

This is looking like a really good start and it sounds to me that you're approaching the methodology really sensibly. Remember to play with every slider and button you see to see what it does....that's how I learned most of the stuff I know.