The Forte de Santo Antônio stands at the northwestern tip of the historic centre, looking out over the bay of São Marcos. Parts of the original French fortifications from 1612 are incorporated into the current structure, which was heavily rebuilt by the Portuguese in the 17th and 18th centuries.
The fort is partially open to visitors and offers the best elevated view in the city: from the ramparts, you can see the bay, the mangroves, the industrial port of Itaqui in the distance, and the sweep of the old city's tiled rooftops. It's a quiet spot that most visitors to the historic centre miss, which makes it more pleasant than the busier sights.