A fortress in San Juan, Castillo San Cristóbal was built by the Spanish to protect against land based attacks on the city. The largest fortification, encircling the city of San Juan, is part of the San Juan National Historic Site.
The fortress contains five extremely large cisterns used for the storage of water during the ages of the Spanish Colony and bomb shelters during World War II.
Address:501 Calle Norzagaray, San Juan, 00901, Puerto Rico
Website: https://en.wikipedia.org/wiki/Castillo_San_Crist%C3%B3bal_(San_Juan)