Information about the small lagoon-based Fuerte San Jerónimo (behind Caribe Hilton in Puerta de Tierra) is difficult to come by, perhaps because it’s the only fort in San Juan [1] that isn’t part of the San Juan National Historic Site. Instead, it’s overseen by the Institute of Puerto Rican Culture and managed by the Caribe Hilton, on whose property it now sits. Various sources date its origins to the 17th and 18th centuries. Unfortunately, it’s rarely open to the public.
Links:
[1] http://www.moon.com/destinations/puerto-rico/san-juan