The Gateway Arch and its subterranean museums and galleries commemorate St Louis' position as a major gateway to the Great Plains, and its contribution to the westward expansion of the mid-19th century. The Arch itself is a series of tapering triangular sections, steel-framed, faced with a stainless-steel sandwich. There's a pair of tramways which carry passengers up the arch to the observation room at the top. You can see the small observation windows at the top of the arch. The view from the top would have been wonderful if it hadn't been raining. Oh, well.