At Fort Nelson the ditch consists of a cut, forty feet deep and fifty feet wide. It is revetted on the inner side with a brick arched wall, napped with flints to provided protection against shell splinters. Other forts on the Portsdown hill line had an outer revetted wall of similar construction. At Fort Nelson it was considered that the natural chalk would be sufficiently stable to stand as a vertical counterscarp. The caponiers project into the ditch to allow the guns mounted inside to fire along each section and so clear it of enemy troops.
The main North-east ditch viewed from the glacis in front of the East Demi-Caponier.
The outer counterscarp was originally a vertical chalk wall which has fallen in over many years.
The slight hump in the floor of the ditch is a man-made firing step for a later rifle range.