Castillo San Felipe del Morro sits on the northwesternmost point of the islet of Old San Juan in Puerto Rico. Tunnels and staircases are still part of the interior, bringing back the time when the port provided the keystone to the protection of the Spanish Empire that spread across the Caribbean. The fortress was designed to guard the entrance to San Juan Bay and construction began in 1539. On May 12, 1898, US Navy warships shelled El Morro in a day-long bombardment but just six months later, Puerto Rico would become a U.S. territory with the end of the Spanish-American war. During World War II, El Morro was still an active military post. In 1942, the U.S. Army added concrete artillery observation posts and an underground bunker to defend against possible German attacks.