sketch.jpg
First off my honest apology for using part of your map. but as i'm not native english for me is sometimes very hard to explain.
I like your concept and I think that the hard time for you starts when your texture are TOO DARK.
My personal opinion, from my own experience, would suggest you to use a lighter texture.
Using black doesn't make the drawing darker. you might make so that the darkest color isn't the black. NEVER.
So you still have the opportunity to make the area darker.
As you see from the two palettes in the top corner, your lighter color (picked from your texture in the neutral area, not the higlighted rooms) is placed in my mid-dark palette.
So, in your case.. If you want to go darker, when going deeper i think the challenge is in to balance the light at your best.
If you start with a lighter color you will be able to make it darker as long you go trough the sessions of your dungeon, till the center.
I think also, you shoulnd't just keep a gradual shade, as you see in my sample, i tried to create different elevation using just a multiply and a screen layer, and applying different shades with the pressure of the pen.
If, you don't have a pen to draw, or you are not interested to handle the shades in this why, one option is to create a folder for the shadows and a folder for the lights.
On each folder you can have the usual layers with the blend you preferer as always, but you can play with opacity to simulate the pen pressure and add tiny details and interesting shades.
Also, i think that when the terrain goes very down, also the shadow dropped will be longer, as the source of light will be placed in different positions
But this is another complex thing and i'm not even the right person to explain it, as i'm just an amateur and i never have studied, i just ..xperiment untill it looks ok to me. LOL
So... in short:
Try to start from a neautral medium lighted texture (or apply different trasparencies of light to your actual texture)
and add shadows, not going straight, but with a dinamic layout..
Have different eleation in the same room as well..
Try, if you wish, and let me know how it goes for you!
Hope this will help ya!