The Fort of São Julião da Barra is the largest and most complete military defence complex in the Vauban style remaining in Portugal.

Considered in the past as the most important maritime fortification in the country, it had originally, together with the Fort of São Lourenço do Bugio, the role of controlling access to the port of Lisbon.

In 1549 John III created the position of Master of the Works of Fortification of the Kingdom, and initially entrusted the architect, Miguel de Arruda, with construction of several fortified complexes in overseas Portuguese territories.

[4] At the beginning of the Portuguese Restoration War (1640–1668), the fort suffered its second siege, by troops loyal to the Duke of Braganza, who went on to become King John IV of Portugal.

The fort was transformed into barracks for the French occupation troops under the command of General Jean-Pierre Travot, while the bar of the Tagus River was blocked by the British navy in support of the Portuguese.

On 22 August 1951, São Julião ceased to be a military fortification and was adapted for the reception of members of the government and accommodation for illustrious visitors.