The rules say you can place a Noble on any Kingdom Space on the board. Can you place them on cities if the strength of your army is greater than the City's defense at setup?
You could conceivably take quite a few cities on Turn 1....although I guess you're limited by the total strength of your armies....
Hopefully this is quick enough answer
Barons take turns placing
one Noble who has been assigned troops on any
Kingdom area of the board. Nobles may not be placed in
overseas areas or in the same area as another Baron’s
Noble or stronghold. A Baron may place Nobles in areas
that contain his stronghold or other Nobles he controls.
Those are the rules, so you can place your nobles in empty Kingdom cities. The strength of your army doesn't matter here.