The Norman castle moat clearly visible,though nowadays not water filled.
The wall in the distance is that of the older Roman 4th.century Saxon Shore fort,inside of which was built,the Norman castle.
for details of the Roman fort of The Saxon Shore see http://www.roman-britain.org/places/anderitum.htm
see http://www.castlefacts.info/contentpages/castledetails/castledetails.asp?country=England&countyid=40&county=Sussex&castleid=3836&latitude=50.8188018798828&longitude=0.334199994802475&uin=11108