Roma Plastalina Part 2

My idea here was to sculpt the negative space around a bunch of building shapes, so the lowest level in clay becomes the top of a building once I pour plaster on and flip the thing over. This is my first layer:

I added about 5 layers total, each time adding and blending little snakes of clay  to incrementally build up the walls which become "streets" 
The clay I'm using is the softest grade of Roma Plastilina available, and I swear it's as soft as toothpaste. It's difficult to keep the edges crisp on these walls because they are about 1/8" wide and I sculpted this with a wooden knife and a knitting needle.

I like the clay negative shown here much more than this plaster relief. I just wanted to sculpt the negative space as an exercise, and design it in layers that change gradually.