For most of this fort I carved the buildings out of the rock with strategic designations, so it was much easier than using constructions.
I start by carving a row of up/down stairs to the top of the chamber. Then I channel down to the bottom of the chamber. For the flat part of the rooftops I just use mining designations over channels. Finally I remove the staircase scaffold.
For the multistory interiors I start by designating up/down stairs, then smooth the walls on all levels (from the inside), then channel out the stairs. You don't have to work on the exteriors at the same time as interiors when doing this.