Fort Amherst, in Medway, South East England, was constructed in 1756 at the southern end of the Brompton lines of defence to protect the southeastern approaches to Chatham Dockyard and the River Medway against a French invasion. Fort Amherst is now open as a visitor attraction throughout the year with tours provided through the tunnel complex

The primary purpose of all the Medway fortifications was the defence of the Naval Dockyard. This was largely the result of the Raid on the Medway in 1667 when the Dutch fleet inflicted heavy damages on the dockyard. Defences were planned for the dockyard from 1708 and land was then acquired by two Acts of Parliament in 1708 and 1709. The land was surveyed in 1715 by the Duke of Marlborough. The first plan of defences was an enceinte (ring of fortifications), from Gun Wharf, Chatham, to north of the village of Brompton.

In 1755, the 'Prince of Wales' Bastion, 'Prince Williams Bastion', 'Kings Bastion', 'Prince Edwards Bastion', 'Prince Henry's Bastion' and the 'Prince Fredericks Bastion' were all built. These and the ditches, built during the Seven Years' War (1756–63), became known as the Chatham lines and were entered by four gateways with bridges. The fortifications were designed in 1755 by Captain John Peter Desmaretze of the Board of Ordnance and consisted of a 9-metre-wide (30 ft) earthwork ditch and a 3-metre (9.8 ft) parapet.

In 1757, an infantry barracks (for a troop garrison) was built to man the defences.

During the American Revolutionary War (1778–83), the lines were enhanced and strengthened. The strongpoint of the design were two Redoubts - 'Amherst' (at the southern end) and 'Townsend' (at the northern end). Amherst Redoubt later became Fort Amherst. Each was equipped with 14 42-pounders, 10 9-pounders, 8 6-pounders and 2 4-pounder guns.

In 1779, during the construction, workmen found an existing foundation of a Roman building. Several finds, including pieces of Roman brick and tile, were made. Roman coins were also found, including one of the Empress Faustina, and one of the Emperor Claudius. The finds were recorded by Rev. James Douglas, working as Lt Douglas with the Royal Engineers 'North Lincs Militia, who later wrote a book describing all of his archaeological research - Nenia Britannica.

During the Napoleonic Wars (1803–15) the Chatham defences were enlarged and considerably strengthened. Further batteries were added (such as the Cornwallis Battery) and the ditches revetted (lined with brick), to the plans of General Hugh Debbrieg, chief engineer for Lord Amherst. Debbrieg had originally helped in the "Cumberland Lines" planning with Capt. Desmaretze. His plan for the Chatham lines, drawn by Joseph Heath and dated 1755, is kept at the British Museum.

Also in 1802–11, prisoners, mostly convicts from St Mary's Island, were set to work on extending the tunnels and creating vast underground stores and shelters, new magazines, barracks, gun batteries and guardrooms. More than 50 smooth-bore cannon were also mounted. The last building works were completed in about 1820. A maze of tunnels, used to move ammunition around the fort, were dug into the chalk cliffs.