Originally there are two villages either side of a canyon, with a river flowing beneath. Over time these villages grow, advance and become interested in trade. Eventually a rickety wooden bridge is constructed, but this is clearly not enough for trade in goods, and a wealthy merchant commissions the building of a bridge. The bridge is constructed over the course of hundreds of years, notoriously dangerous working conditions contribute to a vision of the bridge as inherently evil, and over time the bridge is completed. The whole thing is made of a kind of limestoney/concretey stone that stands the test of time.
Eventually, the bridge is completed, and it is an enormous entity - half a mile in width, and it extends many miles down. The construction of the bridge is only possible because of small housing and shelter built into the side of the canyon, and soon there is a thriving undercity that exists in these spaces in the canyon walls. The bridge's legs, extending down into the canyon, are also gigantic, providing the stability necessary for this enormous weight.
Over the course of the next thousands of years, the villages grow towards each other, and eventually become one great city, stretching across this huge bridge. Space, however, becomes increasingly scarce, and it becomes necessary to build downwards, towards the base of the canyon. Housing is now concentrated on the canyon walls as well as being built into the bridge itself, and there becomes a difference in society that lives above the bridge and below the bridge.