Throughout history, men have built fortified structures to protect themselves from external enemies. These structures have taken many forms over the years, from simple ditches and walls to medieval castles to huge and complex "star" fortresses designed to protect against muskets and cannon. Citadels are large castle-like structures built in or adjacent to cities. They are designed to defend the city against external attack, and (often) to protect the city's rulers against internal revolt.
During the fifteenth through seventeenth centuries, Spain was a mighty colonial power with cities scattered throughout the Caribbean and Central and South America, providing Spain with (among other things) enormous quantities of silver and gold. These cities were under constant threat of attack, both from European fleets and armies as well as from native uprisings. To protect its possessions, the Spanish built a series of mighty citadels in its most important cities. When properly garrisoned and supplied, these were extremely difficult to capture. Remnants of these Spanish citadels can still be seen throughout the New World - El Castillo San Felipe del Morro, in San Juan, Puerto Rico, is a fine example of Spanish defensive works.