Among other things, Offa built an earthwork wall called Offa's
Dyke, which ran nearly the full border between Offa's kingdom of
Mercia, and the Welsh kingdom of Powys. This was roughly the border
between England and Wales.
The purpose of Offa's Dyke is not really known from history, but
it can be surmised that it was a defensive barrier and was also a
political indication of power and a demarcation of the border.
There is a link to an article on Offa's Dyke below.