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.
The move is still early morning. But if you catch a cloudy or windy day when the heat isn't punishing, going near closing is a great alternative. Fewer crowds, and if you time it right, an epic sunset from the ramparts as your reward for surviving the heat.
Open Monday through Sunday, 7:00 a.m. to 6:00 p.m. On free-admission Sundays for Colombians the entry window shifts, so check fortificacionescartagena.com.co for current hours before you go.
Entrance runs about 38,000 COP for foreign visitors, with reduced rates for Colombian citizens, students, and children. Buy tickets only at the official taquilla. A guided tour is not included in the ticket and is offered separately by guides waiting at the entrance. They're inexpensive and well worth it for the history, but English-speaking guides are limited, so if you want a tour in English it's worth asking around until you find one you click with.