The first stone defence of the River Medway dated from the late 14th century when Queenborough Castle was built.

Chatham started to be used by the Royal Navy as a dockyard in the mid 16th century. There was a gun battery at Gillingham and a blockhouse at Sheerness, but in order to protect the dockyard and ships more effectively, the construction of Upnor Castle upriver began in 1559 and in 1585 a chain was added across the River Medway to the dockyard. In the 1620s, the dockyard was greatly enlarged, and a brick wall was built around it.

In the mid 17th century a new chain protected by gun platforms was placed between Hoo Ness island and Gillingham and work was started to build a new fort at Sheerness. A warship was also stationed off Sheerness as extra protection. After the Dutch attacked Chatham in 1667 and destroyed the partly built Sheerness fort, it was rebuilt in 1669 and two extra gun batteries (Cockham Wood Battery and Gillingham Battery) were also added in 1669 as extra defences.

Although the approaches from the sea were now protected, there was concern the the French could invade and attack the dockyard from the land. In 1756 a line of bastioned fortifications, the Cumberland Lines, were started around the dockyard of Chatham and the town of Brompton, a mile and a half in length. Two extra defences were built at each end, Townsend and Amherst Redoubts. At the beginning of the 19th century Amherst redoubt was greatly strengthened and became known as Fort Amherst. The Cumberland Lines themselves were also extended to the north to cover the enlarged dockyard and the village of St.Mary's. At the same time further Medway defences were added, Fort Pitt (1805-1819), a pentagonal bastioned fort and Fort Clarence (1805-1811), a large brick gun tower with defensive ditches to the south of Rochester Castle.

In the 1870s, in keeping with more modern defence theory, a series of outlying polygonal forts were built around the south, south-east and east edges of Chatham to defend the dockyard. Neither these forts nor the Chatham lines were ever attacked thanks to the ability of the Royal Navy to prevent an enemy from crossing the channel.