Hadrian's Wall, England

Pretty much the finest piece of Roman heritage anywhere in Britain (OK Bath, calm down) – a 84-mile ribbon of masonry that winds across the north of England, almost touching the Irish and North Seas at either end.
It was built between 122 and 128AD on the orders of (you've probably guessed who) the Emperor Hadrian, who, at the time, was having trouble with unruly elements in various parts of his vast empire, and wanted a visible barrier to shore up the northern extremity – the Pictish tribes at the top of Britannia having proved themselves unwilling to be conquered.