Not painful enough, they'd probably die pretty quick. This basement is pitch dark and can be very cold during winter, stomped mud floor and low height so that you can't walk upright. Flooding with raw sewage is common, and when it happens the rats will desperately climb up and seek shelter on the bodies hanging there.

If you do want impalement too, then catch some of the big sewage rats into a cage, tie the cage to their ass then lock the cage to the floor. Soon as the sewage water starts to leak in the rats will try to dig their way up their ass to avoid drowning.

Speaking of drowning, keel hauling where common among the vikings back in the days too. Tie them after a boat and drag them back and forth through the canals in the town. The water is brown, dirty and full of scrap metals dumped over the years that will leave some nice marks on their bodies and infect the open wounds, in a ways that will make salt seem like a preferred choice.,