Parker makes the interesting overall point that we should not see the boundary fortifications as the border where Roman power stopped; the Empire's power was projected in both directions, and those beyond the limes might still be under Roman control (and in later times, those within the limes might not be). He concludes with admiration for the initial success and relative longevity of the Roman project, and sadness that it is unlikely to be repeated (which is a whole other debate, I think). There are some great evocative descriptions of ruins as perceived by today's traveller and resident, and some nice historical and archaeological points (eg the soldiers found dead in their fortress in Germany, killed by raiders but never buried); in general it's an excellent book.
It is let down by the fact that the numerous lovely photographs are presented out of order and without cross-referencing to the relevant pages, and also (I know I keep going on about this) by the use of endnotes, so that relevant and interesting information is buried hundreds of pages from the text to which it refers. I wouldn't mind if this was merely a question of providing precise citations, but the notes have a lot more narrative material. No publisher should do this and no author should tolerate it from their publisher. In these days of advanced technology, there is no excuse for not having proper footnotes on each page relating to the text on each page, as Gibbon was able to do in the eighteenth century. Accept no excuses and no alternatives.