During the building of the defensive lines (1809-16) a tiered bulwark, the 'North-West Bastion', was built on the edge of the Citadel flanking the northern slope of the Heights. At the same time the 'North-Centre Bastion' was built, halfway along the earthworks between the Citadel and Drop Redoubt. As part of the construction of the North Lines a North Entrance was built, between the Drop Redoubt and North Centre Bastion, giving access to the Heights by way of the North Military Road.