Hadrians wall was not as weak as you guys are describing it.

Hadrians wall did exactly what it was suppose to do which was to

a.Mark Rome's final borders

b. Protect Trade

c. Moniter Trade

e. Maintain Law & Order

f. Stop Pictish Raids on Caravans

The fact that it's forts prevented Pictish Invasions for centuries was just an added bonus, since when it was built the Picts where little more then an ancient equivalent to street thugs robbing the local banks.

Hadrians Wall should have been in providing trade+Law bonuses in British Provinces, and perhaps even the possibility of defending at the wall instead of the city (which nobody would have done anyway because of massive starting unrest means moving your army out of your city=losing the city, but would have been cool to do towards the end of the game when it is impossible to lose to the Celts).