Aside from El Morro and San Cristóbal’s iconic turreted sentry boxes, the most enduring symbol of Puerto Rico is La Muralla. Nearly 400 years old, the city wall is composed of rock, rubble, and mortar that wraps around Old San Juan from the cruise-ship piers on San Juan Harbor to the capitol on the Atlantic Ocean. Its iconic sentry boxes serve as a symbol of the island’s Spanish heritage and resilience in an ever-changing world.

Begun by Spanish colonists in the 1600s, the wall took 200 years to complete and has withstood multiple attacks by the English, the Dutch, and the Americans. But what proved nearly impenetrable to foreign attack has been rendered defenseless by modern life. Automobile traffic, pollution, and misguided attempts to preserve it have endangered the wall.

Forty-five feet wide and 40 feet high in some spots, La Muralla is crumbling in places. In 2004 a 70-foot section below the heavily traveled Calle Norzagaray fell, underscoring the urgency of stepping up preservation efforts. It wasn’t the first time the wall’s fragility was made apparent. A larger section fell into San Juan Bay in 1938, and in 1999, a Soviet oil tanker ran aground, damaging the wall’s northwest corner.

When the U.S. Army seized Puerto Rico in 1898, it took over maintenance of the wall and attempted its first preservation efforts. Concrete was used to patch La Muralla, but that only served to add weight to the wall and trap moisture inside it, which weakened the structure through time.

Now a National Historic Site, La Muralla is maintained by the National Park Service, which has been overseeing efforts to repair the wall. Experts have spent years studying the 16th-century methods used to build the structure in an attempt to recreate the magic mixture of sand, water, and limestone used to stucco the wall. Not only is the repair method they’ve developed more effective than concrete, it serves to preserve the wall’s historic integrity. The process is now being used to repair the wall’s beloved sentry boxes. But it’s a painstaking and costly process, requiring the services of specially trained masons, which the Park Service is hard-pressed to fund for large-scale repairs.

La Muralla once had five gates that permitted access into the city, but only one remains today. The commanding red La Puerta de San Juan was built in the late 1700s and is on the eastern end of Old San Juan beside La Fortaleza. Sixteen feet tall and 20 feet thick, the door is best seen from the wide bayside promenade, Paseo de Princesa. Named after La Princesa, a 19th-century prison that now houses the Puerto Rico Tourism Company, the promenade begins across from Plaza de Hostos at Calle Tizol near the cruise-ship piers in Old San Juan.

Glorious royal palms, a view of the bay, the soaring city wall, the city gate, and an outlandish fountain comprising naked sea nymphs and goats are some of the sights along the way. The promenade continues along El Morro, ending dramatically at the point containing the oldest part of the fort. Paseo de Princesa is the site of frequent festivals and events, and you can usually find a variety of vendors here selling piraguas (snow cones), popcorn, and dulces (sweets).

La Muralla endures, along with the fortresses El Morro and San Cristóbal that adjoin it, attracts 1.2 million visitors a year. Chances are, with the help of preservation efforts, it will continue to assert its soaring beauty and cultural significance as the proud protector of Old San Juan for years to come.