San Juan de Ulúa is the massive fortress-palace-prison complex on an island connected to the Veracruz port by causeway, and it is one of the most historically loaded sites in Mexico. Cortés landed here in 1519, and the Spanish began building a fort shortly afterwards to protect the silver convoys that would flow from New Spain back to the Old World. Over three centuries the complex grew into a labyrinthine structure of walls, towers, moats, palaces, and — most grimly — prison cells where political prisoners were held in conditions designed to break them.
The fortress served as Mexico's most notorious prison through the 19th century and into the early 20th, holding everyone from independence fighters to Porfirian opponents. Walking through the cells — some of which were below the waterline and regularly flooded with seawater — gives the place a weight that the views from the ramparts, which are genuinely spectacular over the harbor and the city, do not entirely dispel. The museum inside covers the history from Cortés through the colonial period; allow two hours for a proper visit.