Some wealthy people would like to live there. They have a thing about soldiers.
What you have described is a large industrial centre which eventually rebelled against the Romans. The inhabitants presumably set up shop for themselves. Changed the names on everything and put up signs saying under new owners open as usual.
Which might go some way to explaining the Battle of Catraeth.
This has always been seen as an attack on wicked Saxons taking the country over. But the troops involved came from Edinburgh and included what we would call Welshmen from various areas. Edinburgh is a fair distance from Catterick and would not normally have been affected by the place. Unless it was in competition of course. If the lads of Edinburgh were really going for the Angles why not go for the people closer to the border?
If Catterick was a thriving comercial centre as well as an administrative centre and military centre, as it is today in the latter respect, it would be a prime target for a battle plan that was a year in preparation.
The usual interpretation of the poem is an anti English one but it's also suggested that it is a satire on a vainglorious 'Celtic' aristocracy which spent a year drinking before going off to their deaths.https://en.wikipedia.org/wiki/Battle_of_Catraeth
Wiki tells us that this area of the country was home to the Votadini but as another article shows they were not there. They were kept back by the Yorkshire lads of the Brigantes. I wonder if Votadini has a connection with Woden?https://en.wikipedia.org/wiki/Votadinihttps://en.wikipedia.org/wiki/Y_Gododdin
I would suggest that Catterick was still populated by its original inhabitants from Roman times, or rather their descendants. It's a perfect centre for trade and transport and is set in miles of very good farmland. I can't see them giving it all up to gangs of marauding Saxons coming up from Kent. These were well trained people as the men from the north were to find out.
Another thing to consider is the origin of the square or oblong houses we are told makes these people not Celts who built, we are told, round houses. They had been occupied for nearly five hundred years and obviously picked up on the new architectural styles. Especially if they lived in an army garrison town.
Others would say it was all made up by the Normans so why bother anyway. No answer to that one.