The Fort of Alto do Duque is located 75 meters above sea level at the southwestern tip of the Monsanto Forest Park, in the parish of Ajuda, in Portugal's capital of Lisbon.
Built at the end of the 19th century as part of Lisbon's Campo Entrincheirado (Entrenched Field) defensive ring around the city, with the intention that it, together with the Fort of Bom Sucesso, would defend against attacks along the River Tagus, the fort is now headquarters of the criminal investigation division of the Polícia de Segurança Pública (PSP) in Lisbon.
[1][2][3] Construction of the fort took place between 1875 and 1890, on land that was part of a farm owned by the Duke of Cadaval.
Situated on a hill with dominant view over the downtown area of the Lisbon suburb of Algés and the Tagus, it has an irregular pentagonal shape, largely buried at the level of the land, and surrounded by a moat.
During the period known as the Processo Revolucionário Em Curso (Revolutionary process in progress) following the Carnation Revolution of 25 April 1974, which overthrew the authoritarian Estado Novo regime, the fort was briefly headquarters to COPCON, a military command created by the left-leaning Armed Forces Movement.