If you look at that landscape with a Roman officer’s brain (lead addled as it might be), it makes a lot of sense. The Antonine Wall sits on the narrowest useful neck of Britain, between the Firth of Forth and the Firth of Clyde, so you get a frontier from sea to sea with the minimum amount of digging and building. Bar Hill in particular is one of the highest points on that line; you schlep through bog and up an annoying slope precisely so your fort sits on a ridge with a commanding field of view over the Kelvin valley and the approach routes beyond.

The Romans aren’t thinking “this is the end of the world forever, we’re too lazy to go farther.” They’re thinking in terms of administratively useful lines. A frontier, in Roman terms, isn’t where patrols stop but where taxation and permanent stone architecture stop. They had marching camps and temporary posts further north and they pushed beyond this line in the Flavian period, and they continued to raid and campaign beyond it even with the Antonine Wall in place. But they wanted one clear, surveyable, defensible line they can tie into fleets on both coasts and run roads along. Hence the miserable hilltop with a great view.

It’s also politics. Hadrian had his nice sensible stone wall farther south. Antoninus Pius needed a military accomplishment to put on the resume, so he pushes the formal frontier forward and a new line, new forts, new distance slabs proudly recording how many Roman feet of wall each unit built. From that perspective, the legionary is not merely damp and covered in midges but also being used as a bullet point in the emperor’s performance review.