Watching over Gaia is this 17th-century hilltop monastery, with its striking circular cloister, church with gilded altar, and stellar river views. Requisitioned by the future Duke of Wellington during the Peninsular War (1807–14), it still belongs to the Portuguese military and can only be visited on the 40-minute guided tours leaving hourly between 10.30am and 12.30pm and 2.30pm and 5.30pm.