Another common mistake is the idea that Hadrian's Wall was a fortification.

We've all seen the imaginative illustrations: there's purportedly Hadrian's Wall, looking like a proper Mediaeval fort, with a walkway and crenelations along the top, which the Romans are fighting from.

But there's no evidence for this, and good evidence against it.

First, the Romans didn't fight from inside of forts. When an enemy approached they would pour out of their camp and assemble in the open in their tried-and-true formation, where they reckoned to have their best chance of success. And they had indeed defeated everybody this way. (We call these Roman structures "forts" but "camps" better describes their function.)

Second, though the top stones of Hadrian's Wall don't survive anywhere along it, similar walls in other parts of the Empire do survive intact, and these walls are topped with a row of simple pointed capstones.

What these walls have in common are small buildings spaced out along them, within sight of each other. We know that (at night at least) Romans used fire to send signals along these watchtowers. What we don't know is if they had some kind of semaphore which they used during daytime.

Generally the big forts are well behind, and can send aid when needed.