Construction for Castillo San Felipe del Morro, also known as El Morro, took nearly 250 years to complete. The Spanish began building the fort in 1539 at the very top of Old San Juan. The six levels of the fortress were strategically designed to give the inhabitants an advantage over any enemy ships coming to the harbor. By the time it was completed in 1790, El Morro was already given the reputation of being unconquerable, winning numerous attacks—including one from Sir Francis Drake in 1595 and a 1625 invasion from the Dutch. The fort was never defeated.
Today, the fort is considered a World Heritage Site, and the National Park Service established it as a museum