"I doubt they are keeping us here due to any concern over our well being."

Far from it actually. The people seemed to regard those not in guilds with more suspicion than those in them. The guilds were a transparent way to manage the population and see that they were all kept in check. The guards at the gate were the same.

"I suspect the mean to keep us inside in order to keep an eye on us, see what our true motivations are."

And actually it wasn't a terrible way to maintain order, it just had far too many gaps to be successful.