Pirates began raiding Panama's coast around 1560, drawn by the silver and gold crossing the isthmus on the Camino Real from Panama City to the Caribbean port of Portobelo. To guard the Atlantic end of the trail, Spain ordered the construction of Fort San Lorenzo at the mouth of the Chagres River, with work beginning in 1598 under King Philip II. The fort sat on a promontory overlooking the river's entrance, a natural chokepoint where cannon could command approaching ships. It did not always hold. In 1671, forces under the English privateer Henry Morgan stormed the fort on their way to sacking Panama City. Over the following decades, the fortification was rebuilt and strengthened, only to be attacked again by British Admiral Edward Vernon during the War of Jenkins' Ear in 1740. The cycle of destruction and reconstruction gave Fort San Lorenzo the layered architecture of a place that has been fought over more than it has been lived in.

For a century after the Spanish abandoned both overland trails across the isthmus, preferring the long voyage around Cape Horn, the Chagres River slept. Fort San Lorenzo served as a prison. The village of Chagres shrank to a scattering of huts. Then, in 1849, the California Gold Rush jolted the isthmus back to life. Steamships from the American East Coast deposited thousands of travelers at the river's mouth, where a wild-west boomtown called Yanqui Chagres sprang up on the opposite bank from the old village. Steamboat service on the Chagres cut the crossing from days to roughly twelve hours. But the boom died almost as quickly as it began. When the Panama Railway was completed in 1855, reducing transcontinental travel to about three hours, its Atlantic terminus at Colon became the isthmus's primary port. Chagres, bypassed, faded back into obscurity.

Fort San Lorenzo has been government-protected since 1908, and today the ruins and the village site lie within the 30,000-acre San Lorenzo Protected Area, former Canal Zone territory. In 1980, UNESCO declared the fort a World Heritage Site alongside the fortified town of Portobelo, 30 miles to the northeast, under the title Fortifications on the Caribbean Side of Panama. UNESCO described them as magnificent examples of 17th- and 18th-century military architecture, part of the defense system the Spanish Crown built to protect transatlantic trade. The jungle has been a persistent third party in the fort's history, reclaiming stonework between each human abandonment. Vines push through gun embrasures. Trees root in the parade ground. The promontory still commands a view of the river mouth, but the ships it was built to repel stopped coming long ago.

The Chagres River outlasted every human settlement built along it. Its waters were dammed in 1913 to create Gatun Lake, the vast artificial body of water that forms the central passage of the Panama Canal. The same river that carried gold rush travelers upstream on barges now fills the locks that lift container ships 85 feet above sea level. Fort San Lorenzo watches over the river mouth as it always has, though the traffic it once guarded now enters the canal at Colon, several miles to the east. From the fort's crumbling battlements, the Caribbean stretches north and the Chagres curves south into jungle that has barely changed since Columbus sailed past in 1502. The village of Chagres is gone. The fort remains, less a ruin than a monument to the proposition that geography is destiny -- and that destiny can be rerouted by a railroad.