While El Morro gets all the fame, Castillo de San Cristóbal is equally fascinating and well worth a visit. San Cristobal is bigger than El Morro, and is the largest fort the Spanish ever built in the New World. Spread over 27 acres, it features five independent units connected by moat and tunnel, each designed to be self-sufficient should the others fall.
The fort was built between 1634 and 1790 to guard the eastern gate to what is today, Old San Juan. It was designed to protect against a land attack, making it quite different from El Morro, which was built to guard against attacks by sea.