The two towers guarding the entrance to Portsmouth Harbour are the oldest surviving defensive structures in the city. The Round Tower (1418) and Square Tower (1494) form a pair of stone artillery platforms on the Hot Walls — the seaward face of the medieval defences.
You can climb both for free. The Round Tower gives the classic photograph of the harbour entrance with the warships, ferries and Spinnaker Tower behind. The Square Tower contains a small exhibition and is sometimes used for events; check the boards by the door.