Fort São João Baptista guards the mouth of the River Ave, where the river meets the Atlantic. Built in the 17th century and subsequently modified, the fort sits at the southern end of the beach where the estuary narrows. It is a compact, well-preserved example of Portuguese coastal fortification.
The walk south along the beach from the town centre reaches the fort in about 20 minutes. The exterior walls and bastions can be walked around and photographed freely. The interior has not always been open to visitors, so check locally. The setting — fort on one side, ocean on the other, the river mouth between — is excellent regardless.