Giving definition to much of central Sinop are the very hefty remnants of stone walls. With origins dating back four millennia, these once ran 3km, impregnably ringing the whole city. Several towers still stand, some 25m tall; the most impressive of these overlooks the harbour and can be climbed from Ergül Sokak.
It's topped by a relaxed, open-air cafe/bar but you don't have to have a drink to enjoy the fabulous views.
On the western side of the centre, a considerable length of fortress wall encloses the Tarihi Cezaevi, with large bastions also leading down from there to the north coast. Atmospherically collapsing wall sections continue northeast from here along what would have been the coast before the construction of a new reclamation arm.