Done the Great Wall of China? Try the Great Wall of England. It's arguably the finest Roman monument north of the Alps.
Constructed between 120 and 128 by the Emperor Hadrian, the wall originally extended 117 km from the west coast to the east coast, completely bisecting Britain. To the south lay the Roman Empire. Over the wall, north, were the unconquered painted people -- the druidic tattooed Picts and Scots.
Why Hadrian wanted a wall on such a scale remains unclear. Hadrian's legions were quite capable of containing Celtic exuberance without it. Perhaps it was to keep his army busy. Or a pharaonic yearning for a memorial on a grand scale.