Temple of the Celestial Cult

From publisher blurb:

The acropolis of the Celestial Temple is a simple building, a free standing portico made of marble and stone with steps going up to a domed observatory furnished with a primitive telescope.

The foundations of the portico are a slab of grey stone into which a narrow stairwell runs deep into the earth. Down there, it is rumoured, the Celestial Cult practise astrology and the dark arts.