OLD SAN JUAN, Puerto Rico – Puerto Rico is now a commonwealth of the United States and can count on its protection.

But centuries ago, the tiny island was a province of Spain and stood as the gateway to the Caribbean. Its prime location made it a target for attack by foreign nations, including the U.S.

National Park Service, San Juan National Historic Site

Castillo San Felipe del Morro (Fort El Morro) was built to protect San Juan, Puerto Rico, from sea attacks. Construction began in 1539.

To help fend off such assaults, Spain built two massive forts on the shores of the Atlantic Ocean to serve as a two-pronged land and sea defense system. More than 450 years later, Castillo San Felipe del Morro (Fort El Morro) and Castillo San Cristóbal (Fort San Cristóbal) sit at the edge of Old San Juan as a reminder of the city's protective legacy. Both have been declared World Heritage Sites by the United Nations.

Although San Cristóbal is seven times larger, it is the older El Morro that draws more attention and is Old San Juan's top tourist attraction. It was built to protect San Juan from sea attacks. Construction began in 1539 and continued for more than 250 years.

El Morro was designed almost in the shape of a ship, with 18-foot-thick walls guarding six levels, most of which were equipped to fire on like levels of an approaching ship. The lower levels fired on the waterline of vessels entering the harbor, while guns on levels three and four targeted the decks. Level five guns supported the others and also protected the western side of Old San Juan, while the sixth and top level had guns aimed toward land to protect the city against attack from that direction.

Adding to the near-invincibility of El Morro was a dry moat the length of the landward wall. This allowed the wall to be sunk deep into the ground and made scaling it a virtual impossibility. Even today, the moat is an imposing sight for anyone entering the fort's huge walls from Old San Juan.

Many tourists pose for photographs on the bridge that spans it, and it is easy to imagine the futility of ground troops who might have tried a foot assault.

Back inside the walls, visitors can use ramps, broad steps or spiral staircases to reach the top level of the fort, where long-silent cannons sit, their aim fixed on targets of bygone centuries. Although it is clear the weapons will never fire again, many El Morro guests touch and caress them as if they can feel the big guns recoil from the heat of battle.

Rangers from the National Park Service often must use loudspeakers to warn visitors away from the edge of the walls – there are no manmade barriers – and off the historic cannons.

Next to each cannon are steps that the soldiers used to fire over the walls of the fort before ducking behind the thick walls for protection.

It is this spot that offers some of the most sweeping, picturesque views of Old San Juan, the Atlantic Ocean and San Juan Bay. It is no wonder that Spain selected this vantage point as the keystone of its defense system.

On the same top level, a few yards away, sits El Morro lighthouse. The first lighthouse was built in 1846, and the current one is the third to sit atop the fort. The second lighthouse took a direct hit in 1898 during the Spanish-American War, but the foundation was salvaged. The current lighthouse, which is still used to steer ships into busy San Juan harbor, was built in 1908 atop that foundation.

Back down the steps, to the expansive plaza where Spanish troops drilled for hours, and down the steep ramp that soldiers used to lower heavy artillery, sits the original El Morro tower. Built in 1539, the round turret could hold only four small cannons. History notes that when soldiers fired the weapons during heavy battle, the room would become so filled with smoke that they had to leave to breathe fresh air.

The tower eventually was enclosed as the fort expanded. But a piece of artillery fired from an American ship in 1898 during the Spanish-American War remains lodged in the wall, a visible legacy of the attacks on Puerto Rico and the effectiveness of its defense system.

SAN JUAN FORTS

•Hours: Open daily (except Thanksgiving, Christmas Day and New Year's Day) 9 a.m. to 6 p.m., December through May; 9 a.m. to 5 p.m. June through November.

•Admission: $3 (a $5 combo ticket allows admission for seven days to both El Morro and nearby Fort San Cristóbal). Children 15 and younger are free.

•Tours: National Park Service rangers offer free tours throughout the day, or you can explore on your own. Contact: 787-729-6777; www.nps.gov/saju.