The peninsula of La Mola, lies on the north at the entrance of the harbour of Mahón, and covers an area of approximately 1 Km2.
During the 18th century, the British began the construction of a fort called the fort of St. Anne or St. Anne´s Fort, which was never completed. The British decided to concentrate on enlarging and modernising the existing castle of San Felipe on the south side of the entrance of the harbour.
Menorca was finally handed back to the Spanish, after British sovereignty was brought to an end under the signing of the Treaty of Amiens. The British gained sovereignty in Menorca in 1713, with the signing of the Treaty of Utrecht.
With renewed threats of a possible British invasion, the Spanish began work on fortifying La Mola in 1847. Rivalry between the British and French, during the 19th century was evident in Menorca, which was a shipping route used by both the British and the French, between Toulon and Algeria, and Gibraltar and Malta. The temporary buildings of the new fortress on La Mola, were completed in 1848, a year after they began, but were considered unsuitable and a much more ambitious project was designed, closing off the peninsular from the north coast of La Mola, to the bay of Clot, on the south side, and finally opened in 1852. This was also considered unsuitable and a further project was designed. A more ambitious fort was built, which was to be the final building, and is the fortress of today. The bastion system had become obsolete and was abolished, and a polygonal front was designed with two distinct sections, the terrestrial front and the maritime front. The former defended the access to the fort by means of a beach, and the latter using cannons to defend the entrance of port Mahón. The fortification system was based on the teachings of the French Engineer Montalambert who incorporated casements – spaces covered with brick vaults under a terreplein and parapet, providing secure enclosures for the men, on the interior of a wide dry moat, with levels of angled gunfire from the casements. Another important construction was the hornworks, situated approximately in the centre of the polygonal line, and defended the terrestrial front on right and in front, and the maritime front on the left wing.
Once the fortress was completed, it was armed. This coincided with technological advances in experimental gun design, producing uncertainty within the military industry. The first cannons were the Krupp, 30.5 cm and 26 cm, then the Ordonez 24 cm, and finally the Munaiz 15 cm. Signs of the constant change of material can still be seen in the fortress in the different types of gun emplacements. Later on, during the Second Republic, the latest Vickers guns were installed, two of which are still on La Mola, and one can be seen by visitors during the guided tour as can the underground magazines and loading chambers, from where the projection charges and shells were raised to the firing cannons.