Mamula is the perfectly round fortress visible from any high point in Herceg Novi, sitting on a tiny island at the entrance to the bay. From a distance it looks almost too neat to be real — a stone ring rising out of the water with a single arched gateway.
Boats stop here on the standard Blue Cave tour, and you can visit the courtyard. The recent hotel conversion has restored the masonry and laid out the casemates as guest rooms; opinions in town are divided on whether that's an appropriate use of a site with such a dark history.