The massive structure is Castillo San Felipe del Morro, a citadel protecting Old San Juan since the 16th century. Construction began in 1539 by the Spanish Empire; its design evolved over 400 years to defend the entrance to the harbor. El Morro features six levels, including dungeons, barracks, and an intricate system of sloping walls and high, thick fortifications using local limestone (mampostería). Key to Spanish defense against naval attacks by the English (Drake, 1595) and the Dutch (1625); never successfully breached by sea. Became a U.S. Army post after the Spanish-American War in 1898. Now an essential part of the San Juan National Historic Site, a UNESCO World Heritage site since 1983.