The Sé 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. It 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, in a city that had just been taken. Later centuries added a Gothic ambulatory and a Baroque interior. Earthquakes in 1344, 1531 and 1755 each forced heavy repairs, and every repair was made in the style of its own day, so the building now reads as a stack of periods rather than one design.
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 Christian church. Inside, the dim Romanesque nave gives onto a lighter Gothic choir. The most photographed thing about the building is outside it: tram 28 passes right across the west front, and the frame composes itself every few minutes.