Considered one of the main historical monuments of Tortosa, together with the Cathedral and the Royal Colleges, the Castle of Sant Joan, known as the Castle of La Suda, is a key witness to the Andalusian past of the city. Also preserved here is the only open air Islamic necropolis in Catalonia.

It was built in the 10th century under the Caliphate of Abderraman III, on top of an ancient Roman Acropolis. Despite the height, a huge pit was excavated within the enclosure down to the level of the Ebre river to provide the fortress with water.

After the conquest of Tortosa by Ramon Berenguer IV in 1148, the castle became a prison. It was owned by the House of Montcada and by the Templars and was the favourite residence of King Jaume I. In addition to being a Royal Palace, it has also been a Court of Justice. It currently houses a parador de turisme.

In addition to the very important Muslim cemetery, the layout and the foundations of the walls and the well have also been preserved from the Islamic period. This consists of underground galleries which house a mill and two ovens.

The medieval legacy can be seen in some Gothic windows, the gunpowder store and the parade ground. However, it passed unnoticed as, in the 15th and 17th centuries; the Castle underwent many modifications for defensive reasons.