Great drawings. Ascension is correct. You're going to want to work on the line work to make sure you have good contrast and remove all of the "dirt" from your original drawings. Once done, put it on the top layer of your stack and make the blend mode Multiply. I did this same process for my Haibianr city map (link) and I'll make a suggestion from my experience with that one: make masks! Take a hard-edged brush (pencil) and color in water, land, buildings, trees, or other distinctive areas with it on separate layers. When you're done with that you can use those as selection tools/masks/whatever to keep your drawing clean. If you're planning on putting a bevel on the buildings then I suggest using shapes or square brushes or something to keep it crisp...I didn't do this on mine and it shows. Good luck with these. Done right I smell featured map.
M