A mixed lesson - Hadrian decided trying to conquer the north wasn't worth the effort, so built his wall in the 120s
20 years later Antoninus Pius reckoned the Caledonii north of the Forth-Clyde line were the real trouble,
and a wall was need to keep them out - us in the Northern Forest and the rest of the hill country in between
weren't too difficult to manage - with a judicious mixture of sticks, carrots and linkie-hunting
But Antoninus (who never visited Britannia) hadn't got much intelligence (in any sense)
and it was a bit of an embarrassment, the troops soon pulled back to Hadrian's much bigger and better and beautiful wall.
In 208 Septimius Severus had another go at re-fortifying what was left of Antoninus's embankment (which is really what it was),
and invaded the north again, but had to pull out because the Parthians were threatening the other end of the Empire.
So the Antonine Wall was on the whole a bit of a failure - but Hadrian's one was actually pretty successful for 300 years.
Wonder how much the Romans missed the linkie-hunting after withdrawing behind Hadrian's.