The Cathedral of St. James the Great (Sé Catedral de Beja) is one of the oldest buildings still in active religious use in Portugal. Parts of the structure date to the Visigothic period — well before the Moorish occupation — though much of what you see today reflects later medieval rebuilding. The cathedral is relatively plain on the outside, but the interior has several notable features: a Manueline portal, a striking 16th-century nave, and chapels containing some fine Renaissance and Baroque sculpture.
Beja became a bishopric in the 6th century, making this one of the oldest dioceses on the Iberian Peninsula. The cathedral's size reflects the town's former ecclesiastical importance — Beja was a more significant city in medieval Portugal than it is today. Worth a look if you are already exploring the old town, which takes no great detour.