High Quality Content by WIKIPEDIA articles! The Gateway Arch, or Gateway to the West, is an arch that is the centerpiece of the Jefferson National Expansion Memorial in St. Louis, Missouri. It was built as a monument to the westward expansion of the United States. At 630 feet (192 m), it is the tallest man-made monument in the United States,Missouri`s tallest accessible building, and the largest architectural structure designed as a weighted or flattened catenary arch.