Mosteiro da Serra do Pilar

Watching over Gaia is this 17th-century hilltop monastery, with its striking circular cloister, church with gilded altar, and stellar river views from its cupola (it's one of the few places where you can glimpse the Ponte de Dom Luís I from above). Requisitioned by the future Duke of Wellington during the Peninsular War (1807–14), it still belongs to the Portuguese military.