Everyone tells you to walk the walls at sunset. Nobody tells you where to get on, which direction to walk, or that the good part is about twenty minutes long and the rest is a warm brick path with a nice breeze.
So here's how we do it. We're on Carrera 7 in San Diego, which puts you at the northern end of the walls without a taxi or a plan. Walk north out of Plaza de San Diego and you hit Las Bovedas. Get up on the wall there, turn left with the sea on your right, and keep going until you run out of wall at Baluarte de Santo Domingo. That's the walk. It's free, there's no gate and no ticket, and the only thing you pay for is whatever you drink at the end of it.
Timing: be up there about 45 minutes before dark. Cartagena's sunsets are quick and the light goes from orange to gone faster than you'd like. Bring water, wear something with grip, and don't expect to have it to yourself. This is the most popular free thing in the Old Town and the wall knows it.
A note on the alternative, because we'd be lying by omission otherwise: our rooftop looks out over the same sky and it's open till midnight, and locals get 15% off up here too, so expect real company at sunset. If you want quiet and colonial hush, we're probably not your spot. But if you've already done the wall walk once, this is the version with a chair.