The first significant structure on the island of Tenerife and the cornerstone of the defensive system in the harbour of Santa Cruz was the Castle of San Cristóbal at Santa Cruz de Tenerife. Only a few of the original building's walls are still visible today, and they are shown in an underground gallery in Tenerife's capital city's Plaza de Espaa. The name of this fortification is owed to the prominent Calle del Castillo.

Under the direction of Governor lvarez de Fonseca, work started in 1575, but it wasn't finished until 20 January 1577. The castle stood out for having a square or quadrilateral shape, with rhombic bastions or points in the shape of diamonds at each corner. Ashlars were used in the lower portion of the walls while masonry was used in the higher portion. These components were supported nearly entirely by the basaltic lava that was common in this coastal region. A cistern for storing water was located in the fort's courtyard. The basin, which had been situated in the parade ground since the previous century, was subsequently moved to the area that is now the Plaza de la Candelaria in 1813, giving rise to the name Plaza de la Pila, where the mayor's and garrison's residences, the reservoirs, and the cistern were previously.

The military engineer Leonardo Torriani advocated some of the improvements that were implemented over the years, although they did not significantly change the organisation. However, the building was clearly in clear-cut circumstances of degradation that culminated in the fortification's demolition in 1928 due to the natural process of urban expansion of the city, new military tactics, and the current international situation. In order to gain room from the sea and create the expansive square that is today known as Plaza de Espaa, which was finally opened in 1947, a significant amount of materials were spread over the location where it had stood.

Remains that were eventually identified as being those of the castle were discovered on June 28, 2006, while the Plaza de Espaa was being renovated. To assure the preservation of the discovery, preventative and protective steps were implemented at the time. Heavy machinery activity was suspended, and the area was fenced off. The building's structure had suffered significant damage when it was destroyed in 1928, according to the technical assessments that were conducted as a result. The discovered remnants, which belong to the eastern fortification and more specifically the northeastern vertex's bastion wall or diamond point, were conserved and turned into a tiny museum display by being incorporated into the ground of the modern Plaza de Espaa. The ruins are about 7.5 metres long and 2 to 3 metres high at one end and 1 to 2 metres high at the other.

The defensive system that Tenerife had, including those that were dispersed throughout the rest of the island and some of which are still standing today, such as the Castle of San Felipe in Puerto de la Cruz or the Castle of San Miguel in Garachico, is displayed in the underground area where the preserved walls of the castle are on display. These castles include those in Santa Cruz, such as San Cristóbal, San Juan, and Paso Alto.

An explanation devoted to the Castle of San Cristóbal itself, from its construction in 1575 to its demolition in 1928, closes the exhibition project, which consists of a gallery made by the Museum of History of Tenerife and the Museum of Anthropology of Tenerife. The renowned Tiger Cannon, which was originally housed in the Canary Islands Military History Museum, is kept there.