Buildings of great power or importance are plentiful in the Warhammer world. Sometimes, this prominence comes from the magical nature of the place, at others, it comes from the resonance and sanctity awoken in the hearts of its defenders. Regardless, a battle fought over such a building will be a slaughter indeed.
A piece of arcane architecture can either be treated as a building or as impassable terrain (the doors are sealed by the defenders). Models that are inside Arcane Architecture are automatically in range of its effect.