What human-made physical feature forms the northern boundary of the Roman emire?

Answer:
As far as I know, there were no large man-made features at the time of the Roman Empire. Most of the boundaries to the north were formed by rivers. Although there was Hadrian's Wall that formed part of Rome's northern barrier, it was quite small - at only 70 miles long, it created a man made border across one of the skinniest points of Britain.
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