Even on a tight schedule, São Jorge Castle (Castelo de São Jorge) earns the hours.
Allow 1–1.5 hrs.
The walls are mostly reconstructed and the 'castle' itself is light on interior, but you don't come for the masonry — you come to walk the battlements over the whole tumbling-red city to the river. The catch: it's pricey for what's there, the climb up is steep, and the view-terrace gets shoulder-to-shoulder at sunset.