Their main purpose was to protect the entrance of San Juan Bay, ensure that Puerto Rico remained under the sovereignty of Spain, and protect Spanish commerce in the Caribbean basin. The system that defended San Juan includes three major fortifications. The first, La Fortaleza (1533) or "Palacio de Santa Catalina" was and continues to be the residence of governors. Fort San Felipe del Morro (1539), perhaps the most distinct fortification, was built at the entrance of the harbor and has withstood the five major attacks to the colony. More Info on La Fortaleza