St. Louis Gateway Arch - World History Channel The Gateway Arch is a 630-foot (192 m) monument in St. Louis in the U.S. state of Missouri. Clad in stainless steel and built in the form of an inverted, weighted catenary arch, it is the world's tallest arch, the tallest monument in the Western Hemisphere, and Missouri's tallest accessible building. Built as a monument to the westward expansion of the United States, it is the centerpiece of the Jefferson National Expansion Memorial and has become