The ruined Spanish forts ringing Portobelo's bay are among the best-preserved colonial military architecture in the Caribbean, and together with the adjacent town they were inscribed as a UNESCO World Heritage Site in 1980. Three main fortifications survive: Fuerte San Jerónimo, which guards the inner harbour and whose massive walls you can walk along; Fuerte Santiago de la Gloria, sitting on a promontory to the west; and Fuerte San Fernando, across the bay on the opposite shore. All three are open and free to enter.
Spain built these defences after repeated pirate raids — most famously Henry Morgan's 1668 assault, in which a small force of buccaneers used local prisoners as human shields to take the town. The forts were rebuilt after that attack, only to be sacked again by Admiral Vernon in 1739. The cannons still point out over the water, and the scale of the fortifications gives a vivid sense of how much silver was passing through here.
Allow two to three hours to walk between all three forts at a comfortable pace. Fuerte San Fernando requires a short boat ride across the bay — local boatmen will take you across for a small fee.