Dick asked who built Hadrian's Wall, and why. And why it was where it was.

Well point one is it was nothing to do with the English, or the Scots, who wouldn't be around for another few hundred years. The builders were a very cosmopolitan bunch of military types, who included native Celts, Romans, Spaniards, Germans, French (anachronism of course), Greeks, Africans and Syrians, among others.

What for? Its use varied over time (it was in use for about 250 years)- from triumphal border marker, customs post, defensive line, immigration control, base for patrols. It served several of these purposes at any one time, though the mix varied.

Why there? It's certain that at most times the area beyond the wall was Roman controlled or at least influenced, but probably wasn't worth the tax money of a permanent presence. It was poor enough to treat almost like the Jordan desert, not worth the effort. It was about the shortest possible route for effective purposes- the Antonine Wall was shorter as a wall, but badly outflanked either side by the Clyde and Forth estuaries, whereas Hadrian's had the easily patrolled Solway Firth on the West, and a straightish inhospitable coastline on the East.