As of now you are unable to build in parts of the stronghold, including the main hall.
While not exactly the same, the fortress bears a strong resemblance to The Lord of the Rings' fortress called Helm's Deep, as seen in the movies and original art.
This is supported by the details of the Stronghold, it has an exterior outer wall protecting another section of the mountain face, where a small stream enters the fortress, there is a large gap in the Wall.
Turning on building in resource rich area's will allow you to build inside the strongholds main hall.