The Forts Of San Juan, San Christobal and El Morro

Shortly after Christopher Columbus discovered PR on his second voyage to the New World, Spain began construction of defenses as this is the first island found to have a deep water port.  Building continued for 250 years as Spain fiercely held on to the island for 400 years, until the US snagged it in the Spanish American war in 1898.  The US military took over the Forts, adding to their defense during WW2 to defend against the Germans and stayed till 1962.   They are now run by the National Park Service.  They are enormous, in fact the largest in the Western Hemisphere.   Really cool to see.