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Ah, here's something I put together for another forum circa 2000:

Here are four major earthen barriers, listed longest to shortest, and there may be more. Offa's Dyke was constructed by King Offa of Mercia... Alfred the Great of Wessex is regarded as the first king of all England. Offa preceded Alfred the Great by a century or so, and attempted a similar unification. Offa also conducted overseas diplomacy, and constructed this earthwork and pallisade that ran along most of the border with the Welsh kingdoms. It was longer than Hadrian's Wall.

Wat's Dyke is two or three centuries older than Offa's Dyke and was constructed by the post-Roman kingdom of Wroxeter. Because it runs along a parallel course for part of its length it was once regarded as, and still frequently misidentified, as a later construction by the Kingdom of Mercia, or a response to Offa's Dyke. It isn't. This one is part of the middle ages, but may have been built when memory of the Roman occupation was still living.

The Wansdyke was the second longest post-Roman earthwork. I've seen it written that it was constructed to make it easier for cavalry to sally forth against attacking infantry, but this doesn't seem likely. It was constructed sufficiently long ago that no record of its construction remains, although it is definitely post-Roman, and it seems to be ambiguous whether it was meant to protect against the north or the south. Even its original length is difficult to determine, as is the question of whether it was completed or used. The name comes from the Germanic deity Woden - Woden's Dyke, due to its sheer size which amazed the invading Saxons - but this long construction is known by many names. Like Wat's Dyke, it is tied into the Arthurian legends, and one of the forts on its length takes its name from Ambrosius.

The Devil's Dyke refers to an earthen barrier almost seven miles long, apparently another remnant of a post-Roman kingdom. The name is also applied to a dry valley, a natural feature, which should show up in the search engine for you as it did for me.

The Grey Ditch is a short earthwork, not really in the same ballpark as the others, but it may have been incomplete because of an interruption (invasion?). All these earthworks are part of an ancient heritage of Europe which includes Glastonbury Tor, Maiden Castle, and similar structures on the continent. It would not surprise me to find that there are more of these post-Roman walls in Britain.