I glued the walls to a piece of mounting board with the sides sealed by masking tape (this is a really important step, I think), and then added a floor made from another piece of mounting board in the inside. I chopped up the sprue and added some bits of plasticard and dowels and glued them to the base as rubble, followed by a scatter of gravel and then sand.
Once everything is dry, I sprayed the whole thing black.
The plan was to spray the black-coated model with my can of grey primer afterwards and then give the whole thing a wash. However, due to its age the grey primer came out of the nozzle a little gooey, and gave a textured paint effect, which was not unpleasing.
When the spray paint is dry to the touch, I painted the steel windows and exposed steel beams in black again and then steel. Finally, I gave the steel structures a wash of Devlan Mud, allowing the excess to pool and stain the surrounding concrete, and then give the rest of the structure a thin wash of Badab Black.
I really like the heft of these walls, and of course the cannon is a delight. Perhaps I should get another box, this time with different figures; at 54mm, they are not really what I need, but I suppose I can always use them as statues.