The Sé looks like a fortress because it partly was one: a squat west front with two blunt towers, battlements along the top and a single rose window, put up in a city that had just been taken. Inside, a dim Romanesque nave gives onto a lighter Gothic choir. Walk from one into the other and the join is obvious. The building reads as a stack of periods rather than one design, because earthquakes in 1344, 1531 and 1755 each forced heavy repairs, and each repair was made in the style of its own day.
It was begun around 1147, in the months after forces under Afonso Henriques and a northern crusader fleet took the city from its Muslim rulers, and it was built on the site of the main mosque. The cloister is where it gets interesting. Excavation there has opened the ground down through a medieval street, an Islamic quarter with a storage jar still in place, and Roman foundations with drains beneath that, all stacked directly under the church. The most photographed thing about the building is outside it: tram 28 passes right across the west front.