Build the dyke

StrategyPage has a book review on the military aspects behind the King Arthur legend, and explains the reason for Offa's dyke (yes, I know: it probably wasn't built by Offa) and other "earthworks" found in England:

the Celtic-Roman folks still used calvary tactics, and the Saxons tended to be infantry.
So the dyke stopped or slowed down raids by horsemen.

But if these cavalrymen were so effective, how did the Germanic settlers cope with and ultimately defeat them?

The heart of Storr’s argument is their adoption of linear earthworks....

The Saxons erected a series of earthworks across the Way, anchored on the flanks in large areas of marsh and forest, effectively securing their settlements and population from surprise cavalry raids, and preventing the use of cavalry by an invading army. The largest of these is “the Devil’s Dyke” with a ditch 15 ft. deep, a bank 25 ft. high, and a length of 7 miles. Storr’s argument is detailed, thoroughly reasoned, and convincing.

Wikipedia article on the Devil's dyke.