The Fortaleza del Castro is a fortress built on top of Monte del Castro. The current fortress dates back to the 16th century, and together with the Castle of San Sebastián and the city walls that no longer exist, it formed part of a strategic network of castles for the defence of Vigo.

It consisted of three walled enclosures. Today, only the first enclosure and part of the second are preserved; the third, which would have been the city wall, has disappeared in its entirety. The first enclosure and the fortress that crowns the top of Monte del Castro, now converted into a viewpoint. The second enclosure is the fortress of San Sebastián, now occupied by the Plaza del Rey and the town hall. Passages and tunnels were found that connected the castles with each other and with the city.

The project to fortify Vigo dates back to the 16th century, due to the frequent attacks on the city, especially by the English. But it was not until 1665, in the midst of the war with Portugal, that fortification work began. It is therefore from the same period as the fortress of San Lorenzo, located in Goián (Tomiño), on the banks of the Miño.

After several attempts to improve them, Vigo suffered the resounding battle of Rande (23-24 October 1702) in the middle of the War of Succession, where the invaders disembarked and sacked the town once again. After this, there were several futile attempts to fortify the town. One hundred years later, in 1809, the castle of Castro Castle was once again the protagonist due to the invasion of the French led by Napoleon Bonaparte, where on 28th March, the fortress was reconquered by the people of Vigo after a hard and heroic battle, after which the French surrendered. From then on, the town was given the title of "Faithful, Loyal and Valiant".

It is said that from the fortress there were several tunnels that connected with different parts of the town, such as the castle of San Sebastián, the historic centre and even the A Guía mountain, located more than three kilometres away, just before the Rande strait. Remains can still be seen in the fortress of San Sebastián and a few years ago remains of them were found in the nearby street of Celso Emilio Ferreiro. Next to the walls of the Castro castle, until not so long ago, the entrance to one of these tunnels was perfectly visible, where it is said that executions were carried out during the Spanish Civil War.