The Fort of Alqueidão, also called Forte Grande, was one of the largest works in the Lines of Torres Vedras. Its earthworks are low rather than dramatic, but the hilltop position explains the system: from here you read the roads, ridges, and fields that Wellington's engineers turned into a defensive trap.
Do not come expecting a castle. Come for the shape of the ground and the scale of the plan. The surrounding Alqueidão circuit links several related military works and is the best way to make the site feel like more than a single ruin.