In 1809 it was the first of 152 forts, redoubts and other defences to be developed as part of three defensive lines between the Atlantic Ocean and the River Tagus that were designed by the Duke of Wellington to protect the Portuguese capital, Lisbon, from possible invasion by French troops during the Peninsular War.

[1][2][3][4] Following the Treaty of Fontainebleau signed between France and Spain in October 1807, which provided for the invasion and subsequent division of Portuguese territory into three kingdoms, French troops under the command of General Junot entered Portugal, which requested support from the British.

In March 1809, Marshal Soult led a new French expedition that advanced south to the city of Porto before being repulsed by Portuguese-British troops and forced to withdraw.

In addition to protecting Lisbon, the defensive lines were also designed to cover Wellington’s own retreat and possible evacuation from a beach close to the Fort of São Julião da Barra, if his troops were overwhelmed by French forces.

It finally saw action in 1846 when, during the so-called Little Civil War, it was the scene of a battle between the attacking troops of the Duke of Saldanha and the besieged forces of the Count of Bonfim.

Built on top of a hill with a Y-shaped design, it consisted of a set of three strongholds, originally separated by three removable bridges, and surrounded by a perimeter wall about 1,500 metres in length.