Calling all armchair generals! Boudica's Last Stand.

[Renatus]

I think the action burst from the valley into the plain

I certainly agree with that.  However, I see no authority for putting the troops up on ridges.  If you could bring them down on to the valley floor, I would have greater sympathy with the site, although I still consider it to be too far north.  Nevertheless, you still have the difficulty that your valley is closed by high ground, which is not what Tacitus tells us.

The centre for the butchery would appear, on Tacitus' account, to be on the plain and, particularly, around the wagons, not at the point of contact, although certainly many will have fallen there and that, in my view, led to a panicked retreat which the Romans exploited.  The discrepancy between the casualties suffered by the victors and the vanquished is, I think, attributable in the main to the fleeing Britons being hacked down by the pursuing Romans.  I think that this is a feature of ancient warfare.

Turning to the Latin, faucibus is the ablative of fauces and it is the latter that we would use when introducing a Latin word into an English sentence.  Fauces is unusual in that it is a word that is plural in form but singular in meaning, which creates a problem if you want to refer to multiple defiles.  In that event, you would have to rely on context to convey your intention.