Hadrian's Wall is a 73-mile set of fortifications built across northern Roman Britain to protect it from raiding Picts. It was very costly to the Empire. But according to a 2008 article in the Telegraph, local Celts found it a highly profitable venture:
Mr MacLeod, senior investigator for English Heritage's aerial survey and investigating team, said: "Having got over the first shock of the invasion and occupation the native population began to see the potential created by the presence of the Roman garrison.
"The building of the wall appears to have provided a boost to the local economy. A sophisticated network seems to have grown up to supply the new market created by the occupation."
He said the survey found photographic evidence of several farmsteads and field networks on either side of the wall which would have adapted themselves to supply crops, livestock and other raw materials, such as leather, to the Romans.