The keep, which is 28m high, was built in the 10C by Countess Nuña Mumadoma to protect the monastery and the surrounding town. The castle was later built by Henry of Burgundy strengthened in the 15C. The keep is surrounded by 7 towers, built on rocky outcrops. From the top of the crenallated ramparts, you have an extensive view over Guimarães, dominated to the south by the summit of Penha.