Construction of the fort began in 1672; it remained in service up until WWII, it's job being to protect the River Thames and hence, the City of London from attack. As the following quote from the official website shows though, it hasn't seen too much action:
"Perhaps because of its strength, Tilbury Fort has never been involved in the kind of action for which it was designed. The worst bloodshed within the fort occurred in 1776, when a fight following a Kent-Essex cricket match left a cricketer and the fort's sergeant dead".http://www.english-heritage.org.uk/server/show/nav.12192 [english-heritage.org.uk]
One of the most interesting things about the fort is it's overall shape, but unfortunately it is very difficult to show this from surface level photos.
99. Tilbury Fort