It lies on an elevation above the Newnill Channel, a tributary of the Rother, whose flat valley, called the Rother Levels, was an estuary within historic times; and even as late as the 18th century the sea was within 2 m.
It lies on the small river Drone, a tributary of the Rother, in a busy industrial district in which are numerous coal-mines, and there are iron foundries and manufactures of tools and other iron and steel goods.
It lies in the valley of the Don, where that river is joined by the Rother, and has communication by water with the Humber.
Within historic times much of this marsh was covered by the sea, and the valley of the river Rother, which forms part of the boundary of Kent with Sussex, entering the sea at Rye harbour, was represented by a tidal estuary for a considerable distance inland.
The Royal military canal which runs along the inland border of Romney Marsh, and connects the Rother with Hythe, was constructed in 1807 as part of a scheme of defence in connexion with the martello towers or small forts along the coast.