The Fortín San Juan de la Cruz dates back to the mid 1600's and was used to defend San Juan Harbor. The fort was designed to create a cross fire with the better known Castillo San Felipe del Morro.
UNESCO, Military - Historic - Forts and Batteries
The fort became a World Heritage site in 1983 and a US Historic place in 1966.
Direct link below.
Links: en.wikipedia.org, www.google.com