Fighting inside the palace

By dualflames, in Horus Heresy

The other day we played the game for the firts time and we found something strange. Traitor got inside the palace, and Imperial army tried to push them out avergonzado_triste But when they tried to attack them inside the palace they were attacking form a fortified area to a fortified area. This point is not covered in the rulebook asustado Does the Traitor army get any benifit (damage reduction) of being attacked? Imperial army lost several units.

The rulebook only says about coexistance fights in a fortified area, but nothing about this.