Lessons learned from this war, which ended with the Treaty of Lisbon (1668), convinced the Portuguese of the need to reorganise their coastal defences by building numerous fortresses and forts along the entire coastline of the country.

Rectangular in shape and in a Mannerist style, it consisted of a battery with cannons, two circular bartizans with a conical cover, three barracks and a guardhouse.

Significant repairs were made in 1831-32 during the Portuguese Civil War and in 1832 it was garrisoned by Miguelist forces and artillery.

In 1940, work finally began to restore the fort but the following year it officially ceased to be considered a military fortification and responsibility for its management was handed over to the Ministry of Finance.

The Fort of Milreu was classified as a Property of Public Interest in 1977 and the Directorate General of National Buildings and Monuments (DGEMN) rebuilt the wall and made other repairs in 1984 and in 2001.