Location of the Frigidus River

[Robert Vermaat]

The idea being if one section is breached the others will still hold.

Yes, that is also the formal explanation.

Perhaps the 'extra' walled areas could be used to house cattle, or perhaps civilians in case of emergency? Or perhaps a reduced defensive force? I've never seen Roman fortresses being constructed in this manner before - i.e. with inner defences like these. To me it resembled Principate forts or cities being 'reduced' by inner walls in later times.