The city of Spirals is an eldritch location built underneath Kurozu-Cho and hidden by Dragonfly Pond. It seems to suck all the inhabitants of the Spiral row house during the final stage of the Spiral curse into itself, turning their corpses into stone which forms its floor. Much about the city of Spirals is unknown, though it may or may not be the source of the Spiral curse.
The city of Spirals is, obviously, made entirely of Spirals which emit a hypnotizing light. One of these spirals is an corkscrew-like obelisk that "completes" the Spiral after everybody in Kurozu-Cho is dead or frozen in time by screwing itself into the Spiral staircase that leads to the city of Spirals.