What's the must-see thing in Cartagena?

Castillo de San Felipe de Barajas sits on Cerro de San Lázaro, about a 10-minute walk east of the walled city. Construction started in 1536 and expanded over 200 years into the largest Spanish colonial fortress anywhere in the Americas. The tunnel network underneath amplifies whispers across 30-metre stretches, a design that let defenders detect sappers without exposing themselves. At 8am the stone corridors still hold overnight coolness, maybe 24°C against the 33°C that the open ramparts reach by 10am. By midday you'll share the battlements with 15-20 tour buses worth of cruise passengers from Manga terminal. Entry costs about 33,000 COP (roughly $8 USD), no reservation needed. You can buy at the gate.

The Ciudad Amurallada is the second priority. It covers about 1 square kilometre between the Puerta del Reloj clocktower gate and the Baluarte de Santo Domingo. Morning light hits the yellow and terracotta colonial facades on Calle de la Iglesia around 7am, and the bougainvillea-draped balconies on Calle del Cuartel still drip from overnight humidity. The palenqueras selling fruit near Plaza de Santo Domingo charge 5,000-10,000 COP for a bowl of mango with lime and salt. Walk this district before 9am or after 4pm. Midday heat with humidity around 87% in June makes the shadeless plazas oppressive. Worth noting, streets near Plaza de los Coches flood briefly during afternoon downpours. They drain within 20 minutes.

Iglesia de San Pedro Claver on Plaza de San Pedro Claver is the third slot. The 1603 Jesuit church honours the priest who ministered to enslaved Africans arriving at the port. The interior runs cooler than the street by 5-6°C, and the courtyard cloister smells of damp stone and old incense. The attached museum holds Claver's cell and colonial-era religious art. Entry is 20,000 COP. You might spend 40 minutes here. The church sits 3 blocks south of the Puerta del Reloj, so it fits naturally after entering the walled city.

Three places everyone lists that you should deprioritize on your first day in Cartagena. Bocagrande's beaches look tempting on the map, but the sand is grey-brown and vendors approach every 90 seconds. Islas del Rosario requires a full day and a 45-minute boat ride from the Muelle de la Bodeguita. The Castillo de San Luis de Bocachica guarded the harbour entrance from 1647, but it sits on Isla Tierrabomba and needs a 30-minute lancha ride. That said, if you have three days, the morning light over the bay from Bocachica's ramparts is the best photograph in Cartagena.