Highlights: The location of the San Juan National Historic Site on the fringes of the Atlantic Midwest Indies is extremely important. Built in 1511, soon after its completion, the Spanish decided to use the island of Puerto Rico as a military base to protect Southwest India and the Empire. In 1533, "Fortaleza" was built next to the port, and in 1539 the fortresses of El Morro and San Cristobal were built on the western and northern shores of the island, respectively. This defense system was continuously strengthened over the next four centuries. This fortress and the Cartagena fortress are extremely impressive throughout the "New World".