The Royal Military Canal

Romney Marsh is mostly below sea level, so drainage has to be carefully managed. The marsh is criss-crossed by artificial ditches, dykes and channels, and it's one of these that Smith has fallen into.

The marsh is girded by one great big almighty ditch, The Royal Military Canal, built in the early 1800s as part of England's defences against a Napoleonic invasion that never happened.

The theory was that Romney Marsh was likely to be the beachhead of any invasion, as it's low lying, sparsely populated, and about 30 miles off the coast of France. Originally, England planned to defend itself by flooding the marsh. This proved impractical, as by the time the marsh was flooded any troops that had landed there would have been long gone. So instead, it was proposed that a big canal would be cut around the marsh. The excavated earth would be used to make an earthwork on the north side, behind which British troops would be stationed - two defensive barrier for the price of one. The canal was also given a strangely kinked route, to allow the British troops a clear shot at any enemy troops that tried to cross it.

It was very much a British Maginot line. If the French had decided to invade, all they had to do was follow the route that worked so well for them in 1066 - land at Pevensey and go around the canal.