Chicago's oldest surviving high-rise, the Rookery, stands in front of you. Completed in 1888 by Burnham and Root, the Rookery is a hinge between two ages of building: its outer walls are thick load-bearing masonry, while an inner frame of iron and steel hints at the skyscrapers to come. The name is older than the building, recalling an earlier city hall on this site where pigeons and politicians both roosted.
Step inside if the lobby is open: the light court was remodeled in 1905 by Frank Lloyd Wright, who wrapped the original ironwork in white marble veined with gilded ornament, all lit from a vast glass ceiling overhead. Compare that luminous court with the heavy dark exterior outside, and you can see the engineering revolution that let Chicago build the first tall city in the world.