During the reign of Henry VIII, the mud banks of the river Medway near Chatham were used for careening ships. Elizabeth I founded the dockyard at Chatham in the late 16th century and Upnor Castle was built a short way downstream for its defence.
The dockyard at Chatham was used increasingly by the Royal Navy during the wars with Spain, and a chain was thrown across the river at Gillingham to provide additional defence. In the 1620s, the dockyard was greatly enlarged, and a wall was built around it. Although this wall had some bastions, it was never really expected to face a full-scale landward attack. The wall was designed more to keep the prying eyes of enemy agents out of the dockyard and to prevent the sailors from deserting.
The importance of Chatham grew steadily during the 17th century, partly thanks to the wars with the Dutch, which put Chatham on the front line. The dutch raid in 1667 highlighted the lack of defences at Chatham. At this time, two shore batteries (Cockham Wood Battery and Gillingham Battery) were built by de Gomme to defend the dockyard. In the beginning of the 18th century, the role of Chatham changed to being that of a shipwright, and various ships of the line were built there, including HMS Victory.
Wars with the French in the 18th century led to concerns about the vulnerability of Chatham. The events of 1667 had led to the fortification of the Medway against naval incursions, but there were no landward defences at Chatham. From 1700, there were various plans to fortify Chatham, but all were refused.
Eventually in 1756, work started on fortifying Chatham according to plans drawn up by the Dutch engineer Hugh de-Beigg. There was to be a line of bastioned fortification around the dockyard of Chatham and the town of Brompton, nearly 2 kilometres in length. Two square redoubts, the Townsend Redoubt in the north and the Amherst Redoubt in the south, formed strong points in the defences.
The lines designed by de-Breigg took advantage of the high ground on the landward side of Chatham, but only had a shallow ditch, and there was just one demi-lune in the entire lines. They were rather long and required a large garrison and over 1000 cannon to defend them.
In the Napoleonic Wars, Chatham became a vital base for the army in case of a French invasion. If a French army did land in Kent, the route to London would be barred by the key crossing point of the river Medway at Chatham.
For this reason, the fortifications of Chatham were improved. From 1805 to 1812 the highest point in the defences, the Amherst redoubt, was greatly strengthened and became known as Fort Amherst. This fort was made up of several independant yet complementary works and formed a citadel.
The various works of Fort Amherst and all their casemates and galleries were all linked together by a system of tunnels that allowed quick, safe access even when the fort was being bombarded.
The lines themselves were also extended to the north to take in the enlarged dockyard and the village of St.Mary's. More barracks were constructed at Chatham, which became a major army base from which counter-attacks could be launched against a French invasion force.
The Medway barrier was completed by the construction of two more forts; Fort Pitt (1805-1819), a pentagonal bastioned fort and Fort Clarence (1805-1811), a large brick gun tower with defensive ditches.
By 1811 the threat of invasion from France had subsided and the Royal Navy was the undisputed champion of the seas, so the fortifications at Chatham were no longer necessary, but they were used in siege warfare training throughout the first half of the 19th century.
In the 1870s, in keeping with more modern defence theory, a series of outlying polygonal forts were built to defend Chatham. 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.
There are surprisingly extensive remains of the fortifications of Chatham today. For the most part the fortifications of the town (the Cumberland lines) are intact, although they are fenced off and overgrown in many places. Strangely, restoration work seems to have taken place quite recently all along the lines - this has possibly been carried out by the army.
The northern section of the lines and the Townsend Redoubt have been built over, but the rest of the lines still exist although access is limited. Fort Amherst is occupied by a historical society who run events and carry out restoration work there. The lower sections of the fort are open daily in the summer.
The historic dockyard at Chatham is a major tourist attraction with three historic ships on display and a number of exhibitions. A school has now been built on top of Fort Pitt, but the sections of the outer walls still exist and Fort Clarence is intact and has been converted into a block of flats.