Calling all armchair generals! Boudica's Last Stand.

[Renatus]

I'm keeping defile, rampart and camp, because SP would want them, and I can't think of a better reason than that.

I'm not entirely sure what you're getting at.  If you're saying that Suetonius built a camp, of course he did; that is what the Romans did.  On the other hand, if you're saying that some translator's use of 'rampart' (which, as John says, is figurative) means that he constructed an artificial barrier on top of an existing feature, you are indulging in supposition that goes far beyond the evidence.  Tacitus is clear; it is the narrowness of the place that provides the protection.  It was narrow enough for Suetonius to be able to block it with his troops and thus leave no space for the enemy to outflank him.  John is right; it is the topography that counts.