Get in touch with your inner historian by visiting Castillo San Felipe del Morro and San Cristóbal, two Old San Juan forts. Originally built to prevent invasion from pirates in the 16th and 17th centuries, the forts, which are at both ends of the historic city, are free to visit. The older of the two, Castillo San Felipe del Morro, aka El Morro, protected San Juan Bay and took nearly half a century to build. When you visit, you'll have a chance to check out all that comprises it, everything from barracks, lookouts, and dungeons to outposts and cannon-firing positions.
What's really cool is that you can get from El Morro to Fort San Cristóbal via a system of underground tunnels. Known as the Gibraltar of the Caribbean, the younger fort is actually the largest fort you'll find in San Juan. Taking nearly 140 years to complete (1634-1771), San Cristóbal is comprised of five different units that come together via their own maze of tunnels. Covering nearly 30 acres, the fort is one of the strongest in all the Americas. You'll be duly impressed by its walls that are incredibly tall. How tall? They are 150 feet above sea level. Hey, if you're going to keep out invaders, you have to make sure they stay out. You can find out more about these two Old San Juan treasures online at http://www.touroldsanjuan.com/forts/.