Izborsk Fortress is the reason most travellers come. It is not polished in the way a museum reconstruction can be polished; the walls feel heavy, irregular, and practical, built for a borderland that was repeatedly fought over by Pskov, Novgorod, Livonian forces, Lithuania, and Moscow.
Walk the perimeter before going inside. The defensive logic is obvious from the slopes: the walls sit above the lake and springs, with thickened approaches where attackers were most likely to come. Inside are the towers, the Church of St Nicholas, and broad views over the Izborsk-Malskaya valley.