The great fortress that guarded the Spanish Main. Go early for the ramparts.
The Castillo de San Felipe de Barajas is the largest Spanish fort ever built in the Americas, a vast stone wedge of ramparts and tunnels above Cartagena, raised to fight off pirates and the English navy.
Yes, go early and walk the ramparts for the views; the tunnels are skippable.
It's a ticketed sight — book online in advance to skip the queue.
Go early, before the heat and the cruise groups, and focus on walking the ramparts for the sweeping views over the old city and harbour.