Military fortifications by their nature are often complex in their history and design, but Fort Amherst is a real challenge for the uninitiated. Unlike its later contemporaries, it is not a regular pentagonal or polygonal shaped fort, but the surviving stronghold of a much larger defensive line that, at its peak, stretched from Chatham to St Mary's Creek, a distance of over two miles. Conceived entirely with the defence of Chatham Dockyard in mind, the first serious fortification was constructed c1755 known as the 'Cumberland Line'. This fortification extended along the landward side of the dockyard and comprised of a series of earthen bastions and ramparts set at regular intervals, the whole being protected by a deep ditch. This was effectively a short-term measure, brought about with the possible threat of a French invasion, and was never really intended to survive the test of time.

During the 1770s large sums of money were spent on a more permanent reconstruction of the Chatham Lines, including an extension to St Mary's Creek and a large casemated barracks. To the north and south, small 'redoubts' originally built of earth and timber to act as strong points were extended, and improved with brick revetting and parapets. Townsend Redoubt has been completely lost, but Amherst Redoubt survives (although not part of the preserved site as it now contains modern married quarters) and was the nucleus of the present fort. The main Lines themselves were also strengthened with brick revetting and parapets. It was during this period that the complex of underground tunnels, found beneath Fort Amherst's Upper Cornwallis Battery, were excavated to provide barrack and storage facilities in the event of a siege. By 1785 the potential capability of the defences was some 190 cannon, manned by an artillery garrison of some 700 men. This figure was never ultimately deployed.

With the rise of Napoleon Bonaparte in the early 19th century came further improvements to the Lines, including part of a proposed south-west extension to protect Rochester. New Barracks at Brompton were also constructed at this time. Following Napoleon's defeat at Waterloo, the Lines were greatly scaled down, and by 1820 only served as a military training ground for large siege operations. In 1850 a new section of Lines called the 'New Ravelin' was constructed as part of a training exercise for the School of Military Engineering, but by 1860 The Royal Commission for the Defence of the UK had decreed that the lines were now totally obsolete.

The importance of the Lines and Fort Amherst was reiterated during the two World Wars of the 20th century, but this was always going to be short-lived. After the cessation of hostilities the whole complex went into a period decline and decay. In 1959 the site was scheduled as an ancient monument, and plans to use the Fort Amherst as a military museum during the 1970s looked promising. Even though several public 'open days' were well patronised, lack of funds ultimately caused this venture to fail.

By the 1980s the Ministry of Defence was looking to sell the land for redevelopment, and this prompted the formation of the Fort Amherst and Lines Trust with the intention to purchase and preserve the site. In less than a year the sale had been completed, and Fort Amherst opened to the public on a regular basis in 1983. Since then much restoration work has been achieved, and additional public facilities have been provided, but the work is ongoing. Several areas of Fort Amherst await restoration as funds allow, including Prince William's Bastion, Barracks and Battery, and Spur Battery to the east.