Just off Piazza Sella amid pleasant hillside gardens stands a stout square tower: the remains of Castello Salvaterra, Ugolino's mighty Pisan fortress. From up here you can appreciate fine views of the old town. To get an idea of what the city looked like before the walls came down, proceed to Via Campidano, where a stretch of the 14th-century northwestern perimeter built by the Catalano-Aragonese remains defiantly in place, complete with towers.