San Cristóbal is the larger of the two San Juan forts — 27 acres of walls, tunnels, and outworks defending the landward approach to the old city. The Spanish built it in the seventeenth and eighteenth centuries on a system of stepped batteries so that if one level fell, the next could fire down on it. Like El Morro it is a UNESCO site and is managed by the US National Park Service.
The walk through is more atmospheric than at El Morro: long dark tunnels, a deep dry moat, magazines and barracks at multiple levels, a sentry box (the Garita del Diablo) jutting out on a spur where, legend has it, a soldier vanished one night. The flat top is a fine place to watch sunset over Old San Juan. Your El Morro ticket covers entry here if used within 24 hours.