Castillo San Cristóbal

El Morro guarded the harbour mouth, but San Juan's weak point was always the land approach, and after an English force took the city from behind in 1598 the Spanish spent much of the eighteenth century building the answer. San Cristóbal covers around eleven hectares, which makes it the largest fortification the Spanish built anywhere in the Americas, and it works as a system rather than a single fort: a main plaza, five independent outworks connected by tunnels and dry moats, each able to hold out if the one in front fell. A sentry box called the Garita del Diablo stands on a rock at the water's edge, reached by a tunnel and attached to a good deal of folklore.

What you actually see

• The tunnels - passages linking the outworks beneath the fort.
• The Garita del Diablo - the isolated sentry box on the rocks.
• The main plaza and ramparts - views over the Atlantic and the old city.
• The dry moats - the layered defences facing the land approach.

When to go

December to April is drier and cooler. Mornings are best, before the heat and the cruise ship groups. The same ticket covers El Morro, so many visitors do both in one day, which works if you start early and walk the ramparts between them.

Honest expectations

There is very little shade and the ramparts are hot by mid-morning. Cruise days fill both forts and the tunnels become congested. The Garita del Diablo is often closed for maintenance or safety. Old San Juan's cobbled hills are hard going in the heat.

Nearby

El Morro is at the other end of the old city; the city walls run between them.