The Twelve-Gear Procession

You enter a circular chamber large enough to house a siege engine. Twelve stone floor tiles form a perfect ring, each faced with a different brass glyph: a spark, a wheel, a tooth, a hammer, a bell, a hand, a flame, a cracked gear, a falling weight, a dark sun, a still point, and a returning spiral. Brass conduits run between them like veins. Above, three enormous gears turn without visible support, filling the room with a slow, uneven tick. Every few seconds, a blue-white pulse travels through one tile, and the central axle answers with a heavy click. A sealed bronze door waits beyond the ring.