If you take my image here and convert it to black and white (CTRL-SHIFT-U), you'll see that the values (or shading) of the images ranges from light to dark. The darker values are closer (generally) and the lighter values are further away.
For a study, try making several overlapping shapes on your canvas. Start with the lightest value then build on top of it, increasing it towards black with each new shape. Do this with no opacity; make solid shapes. This creates the illusion of depth. If you do your values right, you can add a color layer and begin coloring the image.
Distant objects tend to be more bluish and desaturated while nearby objects have more values, and more saturation. In Emerald Cave, I chose to use a greenish atmosphere to give the illusion of an emerald dust that's present in the cave.
I have a commission service on my profile page that will hire me to critique your images and suggest ways you can improve them. It's really inexpensive and I'll even paint over your original image to reinforce one of my points.
For a scribble, it has lots of depth, as does all your works. I always wish it was in 3D format, so I could move the camera inside and look out, or just explore the whole of it inside and out. too bad we can't jump into the imaginations of others.
lol sorry bout that X] Second tie in the past two weeks I've done that to some one. It's Magic: the Gathering. This scene looks like it would take place in the under city where the Golgari are. Just for more info for you lol All and all it's beutifully done. :3