Brochs
Scattered throughout Scotland and now in poor condition are a large number of forts called Brochs. They appear to date from the second century but others may even be a lot older. The Brochs are of Celtic origin and would probably have been built long before the Romans came to Britain.The walls would be between 12 feet and 15 feet thick, and the tower would be as high as 45 to 60 feet. The tower would generally enclose a narrow courtyard and there would be one entrance which was low and narrow leading straight through to the courtyard.
One of the best preserved Brochs - a round tower dating from the Iron Age - can be seen at Mousa in Shetland off the coast of Scotland.