Fort San Felipe del Morro, or Morro Castle, a 16th-century citadel, was designed to guard the entrance to the San Juan Bay, and defend the city of Old San Juan from seaborne enemies. During World War II the United States Army added a massive concrete bunker to the top of El Morro to serve as a Harbor Defense Fire Control Station to direct a network of coastal artillery sites, and to keep watch for German submarines which were ravaging shipping in the Caribbean.