The Castillo de San Carlos de Borromeo sits low on the rocks at the southern end of Pampatar bay, square in plan, with squat stone bastions on each corner and the original cannon still in place along the seaward parapet. Construction began in 1662 after the Dutch raid and continued in stages until the 1730s, with the local limestone and coral-rag walls thick enough to take direct fire from the kind of ship that could enter the bay.
It is the best-preserved Spanish fortress on the island, and visiting it is an easy hour. The walk along the parapet gives the long view across the bay back to the colonial town, and the small museum inside the keep covers the wars of independence at island scale. Open most mornings; entrance fee modest in local currency, free on some Sundays.