Castillo San Felipe del Morro, most commonly known as El Morro, is a large fortress and citadel in the historic district of Old San Juan in Puerto Rico. Commissioned by King Charles I of Spain in 1539, it was first built as a fortified tower in honor of King Philip II, who oversaw its expansion into a hornwork fort by 1595. Over the next 200 years, especially in the reign of King Charles III, El Morro continued to be developed to reach its current form in 1787.