One of the greatest forts in the New World, the Castillo San Felipe del Morro, more commonly known as El Morro, looms over the Atlantic with virtually impregnable stone walls. It is testimony to Spain’s grim determination to defend the island over the centuries and proved an ideal stand-in for the terrifying slave fort in the movie Amistad (1997). Today El Morro is a UNESCO World Heritage Site and part of the San Juan National Historic Site (whttp://www.nps.gov/saju), managed by the US National Park Service.

A short walk from Casa Blanca, the fortress occupies a dramatic spot at the top end of Old San Juan, with six levels of rock-solid defensive positions featuring the distinctive “hornwork” shape, 42m high in places. It dominates the mouth of San Juan Bay, a superb strategic location that made it almost impossible to capture.

Established as a small gun battery in the 1540s, El Morro was constantly expanded and the imposing defences you see today were completed in 1787, thanks in large part to an enterprising Irishman in the pay of Spain, Thomas O’Daly. The fort’s defensive record is certainly impressive and it was captured only once – by the English in 1598.

You’ll enter the fort at the main plaza, surrounded by vaulted rooms that once served as officers’ quarters, magazines and storerooms. Some of these have been converted into a small museum of the site and a video room. From the main plaza you can clamber over the battlements above and below, taking in the stunning panoramas of the bay and cityscape to the east.

The fort is separated from the rest of the old town by a swathe of green grass, which has no official name but is commonly known as El Campo, or just the “Area Verde”. It’s a popular place to fly kites – come here on a blustery weekend and the sky is jam-packed with them. To join in, buy one from the stalls nearby.