Right it also was an attempt to take advantage of the Gallic numbers. The Roman's had repeatedly shown they were the master of the pitched battle due to organization. While the Gallic warriors by themselves were outstanding warriors. By pinning the Roman against a fortified position that had to be fully encircled the Roman forces would have to be spread thin and struggle to maneuver. so it would hopefully turn into into an extended meat grinder. It wasn't too late either. The Roman forces were very close to be overwhelmed during the battle. I think Vercingetorix underestimated the Roman's ability to fortify and stay in his position. In all it was a very ballsy move since Caesar's army was very large too and he was deep into Gallic territory once behind his wall no more supplies would get to him either.

I always wondered why Vercingetorix didn't kick all civilians out of the city early though.