We had an amazing experience staying at one of the prison cells in the Old Mount Gambier Gaol during our road trip last summer. You will realise very quickly that its massively lacking in windows and it's obviously designed like that for a reason. It does gives you creeps and a weird of tension.
The guy owned the place was friendly and hospitable. He gave a good run-through of the history of the place. As its heritage listed, most parts of the building is still left in its original state when it's still in operation, for example the barb wires, doors and floor markings.