Castillo San Cristóbal is a historic fortress located in San Juan, Puerto Rico. Built by the Spanish in the 18th century, it was designed to protect the city from land attacks. The fortress features thick walls, a series of tunnels, and a large moat, showcasing advanced military architecture of its time. Today, Castillo San Cristóbal is part of the San Juan National Historic Site and is a popular tourist destination. Visitors can explore its well-preserved structures and learn about its role in the defense of San Juan during various conflicts, including the Spanish-American War.