This fort defended the Mountain pass of Fonte Boa da Brincosa, the Senhora do Porto valley (also known Senhora do Ó) and the Carvoeira road.
It is a mixed construction, comprising a central redoubt and a forward battery, corresponding to one of the most elaborate constructions of the 2nd Line. The access to the battery area is by a tunnel excavated in the rock and there are still part of the original steps. This is a unique structure on the Lines of Torres Vedras, along with the walled access to the battery. It combines rock excavation with a stone wall, making it one of the most noteworthy part of this Fort. Facing the control and defence points of Zambujal Fort is the battery, where the two artillery pieces in the fort’s arsenal were sited.
This structure was also surrounded by a moat and was connected to the access by a palisade, detected in the archaeological excavations.
The platform where the pieces of artillery were placed was garrisoned with four embrasures in the parapet and a wooden platform also detected in the archaeological interventions.