The moats,snaking around this area,were defences for the gun emplacement,in the middle distance,of this shot.The 19th century fort,called The Coalhouse Fort,is visible in the upper left,of this image.see http://en.wikipedia.org/wiki/Coalhouse_Fort
NB:Just to the right,of the trees on the horizon,in the middle of the image,can be seen a bright flame.This is from the oil/gas terminal at Shell Haven,about 10 miles away ,to the east.