I see no real reason to discount the existence of the battle itself. Certainly, taking their own sweet time about actually forming together into a league is consistent with what we know about Celts from both Gaul and southern Britain. (This assumes that the Caledonians or Picts were in fact Celts, which I believe is still being debated.) Likewise, their leadership being a group of chieftans arguing policy and strategy out in some sort of council while the men each follow their own leader is also in keeping with Celtic culture and practice.

I can’t locate any of the possible battle sites that Pectinarius mentions in my British road atlas, but something not too far from the coast might make the most sense. Tacitus mentions that Agricola had sent the fleet ahead while he advanced with the army. That implies to me that he was using the fleet as some sort of support. Culloden sounds like it might be a bit of wishful thinking on the part of someone, but then again it wouldn’t be odd for the best places for combat in Scotland to be used more than once.

There also isn’t any real reason to completely discount the existence of Calgacus. The name doesn’t seem that out of place with Celtic naming practices. There were plenty of prisoners taken and the battle turned into an utter rout with the defeated Caledonians scattering into the woods. Small wonder if Calgacus was never heard from again. His speech, of course, is a complete fabrication. This is Tacitus’ big chance to hit his readers with his moral lesson. Of course, this is nothing unusual and every ancient historian put words into the mouths of his main subjects. Even Thucydides reports speeches that couldn’t possibly have been recorded anywhere.