The rooms to the north of the main hall were the dwarven rooms for dignitaries and such. The rooms to the south of the main hall were for visitors to the dwarven realm. The main hall could be used for diplomatic ceremonies, but most of the time it served as a market for surface dwellers and the dwarves to set up stalls and sell stuff to one another. While it was constantly inhabited, the population rotated pretty frequently.
I didn't want this level to be inhabited by a particular group, i.e. this isn't an orc hold, or a bandit lair. I want it to be a bit of a mix, probably with a fair population of vermin and a few undead. I'm going to have some orcs in the vault level, having just raided part of it, and they'll be coming out of the vault when the PC's arrive.
What about the main hall? A big nasty thing? Or empty? I'm leaning toward empty, or at least devoid of monstrous threats, especially as the vault sub-level will have some tough encounters. Well, as I'm going for a semi-oldschool feel, lets roll for it. Using the tricks charts from OSRIC (pg 155-156) I came up with an antimagic alter. That kind of makes sense actually. You wouldn't want magic used during diplomatic negotiations! Lets go with that.