More photos of San Juan, Puerto Rico

The old city, which consists of 400 restored buildings from the 16th- and 17th-century Spanish colonial period, features a number of plazas, shops, historic buildings, nightclubs and art galleries.

The walls of the city are up to 42 feet tall and 45 feet thick, according to The Akron Beacon Journal, giving San Juan its nickname of La Ciudad Amurallada, the walled city. Most of the remaining walls were built between the 1760s and the 1780s, according to the National Park Service. Back when walls completely surrounded the city, people had to enter or exit through a series of doors. Only one example of these portals, the “Last Remaining Door,” still stands.

Keeping with the theme of defense, two main forts, Castillo San Felipe del Morro (El Morro, below) and Castillo San Cristóbal guarded the city during colonial times. El Morro sits on top of a promontory overlooking the entrance to San Juan Bay. Spanish engineers laid the foundation in 1539, but it wasn’t complete until 1787, according to the National Park Service. The six-level fortification offers ocean views from 60-feet-tall walls and has a lighthouse that was rebuilt after U.S. Navy ships destroyed it during the Spanish-American War.

Castillo San Cristóbal (below) is at the eastern gate of the walled old city. Construction started in 1634 and finished in 1790. According to the park service, it's the largest fortification the Spanish built in the New World. It was designed to protect the city from attacks by land.