The most impressive post Roman fortification is this...http://en.wikipedia.org/wiki/Wansdyke_%28earthwork%29

There has been very little archeology done on it, but most historians date it to the pre Anglo-Saxon period because it's name indicates they had no idea who made it-they just called it after their god Woden. It is really quite spectacular on it's western side, up in the Wiltshire downs.

Unfortunately the period between the Romans and the early English is very badly sourced for written materials AND archeology. Because they would have been occupying Roman era buildings, and their archeological trace is very slight (no coins were minted for example, potteries stopped production, foreign trade massively declined), lots of it has been destroyed or lost by people digging in to find the Roman stuff.