The Old Cathedral sits on the slope between the lower town and the university, and its military appearance — crenellated walls, arrow slits, no spire — makes it look like something between a church and a castle. That was deliberate: it was built in the 1160s when the Reconquista was still ongoing and the south of Portugal was not yet secure.
The interior is relatively plain compared to later Portuguese churches, which is part of its appeal. The Gothic cloister, added in the early 13th century, is serene and beautifully proportioned. The main portal's carved tympanum depicts the descent from the cross. Entrance is inexpensive and the building is rarely crowded.