I finished modeling my "turtle city" piece in SketchUp.
It's 11 layers of 1/8 in thick material. I've got a view of all the layers in the next image. It's a little hard to see because the buildings in each layer are fairly elaborate.
The turtle is about 3' wide, so the whole piece is about 5' tall.
The next step is to make cut files from this model, which will be a challenge, since my cut bed is only 24x12.