The largest fortification built by Europeans in the New World features dungeons, a labyrinth of tunnels beneath an enormous stone fort, and massive rock walls that fall 150 feet straight onto the rocks of a rugged shoreline. No wonder Castillo San Cristóbal in San Juan National Historic Site was chosen for a scene in the next "Pirates of the Caribbean" movie.

San Juan National Historic Site, located in San Juan, Puerto Rico, includes Castillo San Cristóbal, Castillo San Felipe del Morro, and Fortin San Juan de la Cruz (known locally as El Cañuelo), along with bastions, powder houses, and three fourths of the old city wall. These magnificent fortifications were built by the Spanish beginning in 1533; work on La Fortaleza and a tower at El Morro began in 1539, and continued for more than 250 years.

The big attraction for Disney filming crews earlier this month was a historic ramp entrance to Castillo San Cristobal, which stands guard at what was once the eastern gate to the walled city of Old San Juan. Construction on San Cristobal was begun in 1634 and finished in 1790, although modifications were made well into the 18th Century. San Cristóbal guarded the city from approaches by land; its better-known sister fortification, El Morro, was designed to protect the city from attacks by sea.

Covering 27 acres, Castillo San Cristobal is the largest fortification built by the Spanish in the New World, and the first shots of the Spanish-American War were fired here. A labyrinth of tunnels crisscrosses the area deep beneath the huge structure, which also includes dungeons and outside walls that fall 150 feet straight down onto the rocks of the north shore of the island. According to the park staff, "If you love to photograph spectacular views, San Cristobal is a goldmine for you, with its panoramic view of the coast from Old San Juan to Condado and beyond."

According to information from the park, "The historical ramp at Castillo San Cristobal was chosen by the production group for filming a 17th century carriage, led by two horses, as it was entering the main plaza of the fortification. The ramp, measuring approximately 100 feet long, is part of the original design of the fortification when it was built on the hill which gives San Cristobal its name. The ramp was then used for facilitating the transportation of equipment, ammunition and supplies to the soldiers."

In current times the ramp provides one of two visitor entrances to the site, so the additional access point allowed visitation to continue during the filming activity. Work to set the stage for the scene began on September 8, but any fans hoping to catch a glimpse of Johnny Depp and Penélope Cruz were disappointed. According to a park spokesperson, "A few movie extras, playing minor roles, were seen at the fortification dressed as sailors of the epoch. Depp and Cruz were not at the San Cristobal shooting since they both were filming on the same day at another local site."

“Pirates of the Caribbean: On Stranger Tides" is expected to be shown in movie theaters beginning in May 2011.