San Cristóbal (Calle Norzagaray at the entrance to Old San Juan, 787/729-6777, daily 9 a.m.–6 p.m. Dec.–May, daily 9 a.m.–5 p.m. June–Nov., $3 adults, $5 for both forts, $2 seniors over 62, free for children under 16, English tours at 10 a.m. and 2 p.m.) is the large fortress at the entrance to Old San Juan  by Plaza de Colón . Before it was built, two significant attacks from land—first by the Earl of Cumberland in 1598, later by the Dutch in 1625—convinced the Spanish that protecting the walled city from attack by sea alone was not adequate.
San Cristóbal’s construction began in 1634 and was completed in 1783. The fort eventually encompassed 27 acres of land, although some of it was destroyed to accommodate the expanding city. The fort’s defense was tested in 1797 by another unsuccessful attack by the British.
After the United States won the Spanish-American War, it took control of the fort and used it as a World War II observation post. Today, a section of the fort is open to the public, who can wander freely among its intriguing array of tunnels, ramps, stairways, batteries, magazines, soldiers’ quarters, and turreted sentry posts.