In the Kalemegdan park / Belgrade Fortress there's also a bunker. This is a bunker Tito ordered to be built, and it was constructed in early 1950s. It has two gun ports (now closed), and it is today an open museum that you can visit.
A long tunnel connects the two areas supporting the gun. There's a few additional rooms for storage and crew under the guns and along the long tunnel.Here's the entrance: