I'm not going to quibble about front or rear, it's the south-eastern corner tower of Cooling Castle. Correctly mentioned that it was built for the Cobham family in the last quarter of the fourteenth-century as a defence of the southern side of the Thames Estuary. Then the marshes to the north were not so well developed and an open water course extended from the Estuary up to the Castle.
Masonry has fallen from the eastern tower of the main south gate, but this is in the process of being restored at the moment.
You have it John Walker