On the southern side of Old Town's most famous street, this fortress was constructed in 1502, but much was changed and altered over the centuries. It lies on a steep hill over the mouth of the Río Ozama, and, from this point, the Spanish launched conquests of Jamaica, Peru, Mexico, Cuba, and Colombia. Still standing today, it remains the oldest colonial military building in the New World and is now open to the public. Among the walled buildings here stands the Torre del Homenaje (Tower of Homage), evoking a Spanish castle with walls 2m (6 1/2 ft.) thick and a crenelated tower. If you climb to the roof, you'll be rewarded with a 360-degree view of the city.

Other attractions include the excavated ruins of another fort from 1502; the intact wall of Fort Santiago, the first line of defense; and an old arsenal where gunpowder was once stored. In the courtyard beyond the main gate there is a statue of González Oviedo, author of the first History of the Indies. He also commanded the fort from 1533 to 1557.