From the outside, it appears to be a small rook-shaped tower, but the true Basilica is a vast subterranean cathedral beneath, decorated with stained glass windows depicting Hitomi's haigography, backlit by continual flame spells designed to create the illusion of sunlight. This level also contains offices for the High Priest. and presumably also one for the top secular leader.
Below this are three distinct wings, each with a separate spiral staircase leading down. One contains artificing laboratories, one is dormitories for the guards, and one is a rectory for the priests of Sirius.