TQ7668 : Old Fortifications, BromptonRe this fortification: It is officially known as the Couvre Porte (although it it known locally as 'Cats Eyes') and it was one of several defendable gateways through the Lines. It was built in the early 1800s as part of the repairing and modernising of the Lines during the Napoleonic wars.
More information may be found here
http://www.kenthistoryforum.co.uk/index.php?topic=7641.0, and if you have an interest in the area the Brompton History Research group meet in the King George V pup between about 7-9pm on the 4th Monday of the month, and welcome questions/interest from anyone that comes along.