King Offa was King of Mercia from AD797 to AD796.
He was attacked by British people living in what we now call Wales.
He organised the building of a large earthwork. It marked a boundary so it was clear when the Britons had invaded his Kingdom.
Named Offa's Dyke, it was an eight metre high mound of earth which ran in a rough line north to south for 80 miles.
We cannot be sure how useful the dyke proved to be, but it certainly showed how organised and powerful King Offa was.