Fort San Cristóbal's 27 acres were built to protect the city of Old San Juan from land attacks. It doesn't receive as many visitors, or as much adoration from travel guides as the city's El Morro fort, but recent visitors are equally amazed by the majesty and significant history of the site. Travelers insist you won't be disappointed if you spend an hour roaming the fort's ramparts.

The fort's stellar shoreline views (not to mention the occasional iguana) draw travelers to the sprawling site. Remember you're in the Caribbean and this is an old, crowded tourist attraction: If you want to be most comfortable during your history lesson, visit San Cristóbal in the early morning or late afternoon.

Both the Fort San Cristóbal and the iconic El Morro dominate the San Juan Harbor, making up the San Juan National Historic Site. You should plan to stop by San Cristóbal on the day you visit El Morro, especially considering that you can pay one fee for access to both sites — both are open daily from 9 a.m. to 6 p.m. Admission costs $7 for visitors 16 and older. Visit the National Historic Site's official website for more details.