I wouldn't go so far as to say the wall is incidental to this book, but it is fair to say its scope is rather more wide than the title would suggest.
Moffat begins the story logically enough, with Caesar's initial voyages across the Channel, followed about a century later by Claudius' full invasion. In fact, there is much more detail than I expected about the initial conquest of Britain, and it was certainly informative - I hadn't realised, for example, that the initial push went well into what is now Scotland, beyond even the eventual line of the Antonine Wall, although it would be difficult to describe the situation as stable. Hadrian adopted a policy of limitation and set in place a number of boundary works, including in Germany and in northern Britain (which he visited in 122). The notion that a colossus of Hadrian may have stood at the mouth of the Tyne is new to me, but it's an interesting idea.
However, after Hadrian's death, the new emperor Antoninus pressed northward once again, and set up a new boundary across central Scotland, turning Hadrian's Wall something of a folly in this period. In fact, Moffat gives the impression of a much less settled state of affairs than the history I received at school, with continual border skirmishes, accelerating in frequency towards the final days of the Roman province.
After Rome, of course, many stones were taken from the wall over the centuries to assemble local buildings, including Carlisle Cathedral, Hexham Abbey, and many local farmhouses. Interestingly, Moffat records that in fact the wall possibly stood quite high until as late as the eighteenth century and the period of the Jacobite rebellion when it was robbed out by the British army constructing the foundations of the Military Road (the B6318 which runs more or less parallel to a long stretch of the wall).