El Morro is the great hexagonal Spanish fortress at the tip of Old San Juan, six storeys of golden sandstone walls rising 140 feet above the Atlantic. Construction began in 1539 and continued in fits and starts for the next two centuries; the fort defended the harbour entrance against the English, the Dutch, and assorted pirates, and is now a UNESCO World Heritage Site managed by the US National Park Service.
The visit is partly inside, partly outside. You climb through casemates, vaulted barracks, the dungeon, and the chapel, but the best parts are the ramparts — the open lawn approaching the entrance, the line of garitas (sentry boxes) along the seaward walls, and the views down to La Perla and out over the harbour. Admission is a single ticket that also covers Castillo San Cristóbal. Allow at least two hours. Bring water — there is no shade on the ramparts.