The Lenses of Heaven

From the introduction: Ancient Saaru stood at a confluence of astral paths so dense, travellers (or invaders) could arrive from anywhere. To protect the city, the gods of Saaru gifted it with a pair of magical lenses. Set in the heavens at a place they named Tlarba, the lenses concentrated the astral paths away from Saaru, allowing gate-keepers to intercept any would-be uninvited arrivals. Over time, many chose not to continue to Saaru at all, and Tlarba grew rich - and strange.