San Juan has spent most of its long history behind impenetrable walls. The oldest city in Puerto Rico, was built in the 1500s. It's one of several walled cities used by Spanish people of the Puerto Rico. Spanish boats would stop in San Juan before continuing onto the unknown lands, and on the way back to Spain the floating treasure chests would deposit gold and other valuables in San Juan’s “La Fortaleza”.  It was to protect it from the greedy European nation.

San Juan has never been left alone for a long time. Sir Francis Drake tried to raid La Fortaleza for England. He got attacked by San Juan's strongest defense, then the British attempted another invasion three years later. In 1621, the Dutch managed to burn La Fortaleza, but another fort, El Morro, stood strong and undamage. It was only until the 20th century unitel the United States bombed the city during the Spanish-American War. Then Spain lost the grip on San Juan and Puerto Rico became a United States territory. 

 This photo of El Morro is from this website:

  El Morro, in Old San Juan