Try to add more variation to your building. Think of adding different prison cells (cobwebs, missing paintings, double beds, toilets, itemframes?)
Also try to make your prison more like a cylinder. Use a 6 3 1 1 3 6 3 1 1 3 6 outline to make it look like a cylinder. Also add variation to the blocks you use. You're working with stonebrick, why not try to use stonebrick variants? (mossy, chiseled)

Nice idea though. An environment could be nice too.