San Juan National Historic Site is one of eight official park sites outside of the 50 states and DC. The site consists of two large fortresses located west of San Juan. Construction on the older one started in 1539 and final changes were made in 1940 when the U.S. Army added rooms overlooking the waters where soldiers kept watch for U-boat activity. The second fort, started in 1634 was designed to repel land attacks (the original fort protected against attacks from the sea) and was built after successful raids on San Juan by land.

The first fort - Castillo San Felipe del Morro - cascades down a hill from high ground on a peninsula looking out to the Caribbean. It consists of at least five levels, with cannon located on each level and designed to damage different parts of ships (masts, hull, sails, etc.). The second fort - Castillo San Cristobal - was built with 20-foot thick walls and could protect the local population from invading forces.

Both forts have small gift shops but no video presentations. However, many of the rooms that are open to visit have very detailed displays covering some historic period of the fort and the world around it.