Built to ward off pirates and foreign invasions, the fortress was enlarged throughout the colonial period until it became the massive work you see today. After independence, it served as a prison noted for its harsh conditions and for the famous people incarcerated there, among them Benito Juárez. It's a formidable example of colonial military architecture, with crenelated walls projecting straight up some 11m (36 ft.) from the water's edge, and bastions at each corner. Inside is a large courtyard, with wide ramps along the inside walls, storehouses, barracks, and old prison cells. English-speaking guides are available at the entrance. A cab from downtown will cost 50 pesos.