Castillo San Felipe de Barajas

One of the largest and best preserved Spanish fortresses in the Americas, and genuinely impressive in person in a way that photographs haven't quite prepared you for. The scale of it — the tunnels, the ramparts, the view over the city from the top — stops people mid-sentence. Built by the Spanish in 1536 and expanded over the following centuries, it was designed to be impenetrable and largely succeeded. Go early in the morning before the heat and the tour groups arrive. An hour at Castillo San Felipe first thing in the morning is one of the better ways to start a day in Cartagena.