In 1709, General Stanhope sent Brigadier Petit and military engineer Durand to Menorca, who soon began the works to expand the Castle, while the British Royal Navy set up the arsenal and the Naval Hospital on the Isla del Rey, in the port of Mahón.
The layout of the Castillo de San Felipe was, after the extension, as follows:
A central nucleus: It went from four bastions to eight vertices.
A fortified line: From the port it reached the Cala de San Esteban.
Some exterior forts: They contributed to the joint defense of the fortress.
According to the principles of Vauban, every external position had to be dominated by the interior. The central nucleus of the castle, therefore, dominated all the exterior works that surrounded it.
This imposing fortress had another device that characterized it, particularly reinforcing it: the underground galleries, which allowed communication from the center to the first line and, from them, the moat was beaten through the embrasures, also pulling the counter-mine galleries from them.