Built at the end of the 19th century as part of Lisbon's Campo Entrincheirado (Entrenched Field) defensive ring around the city, the fort is now a maximum-security prison.

The forts were equipped with modern artillery pieces and connected by roads and by telephone and telegraph networks that were quite advanced for the time.

At 241 meters above sea level, the highest point in the Monsanto hills, the fort, at the time, allowed total visibility over Lisbon and the River Tagus.

During the early days of the First Portuguese Republic there were numerous disturbances and strikes in Lisbon and the fort was used to detain strikers in 1912.

[5] Re-afforestation of the surrounding area to create the Monsanto park took place in the 1930s, with tree planting being carried out by the prisoners.