With the attention given to the billions of dollars that would be needed to put up and maintain a wall between the United States and Mexico, something that should come to mind is the boarder between U.S. and Canada. (officially known as the International Boundary). Some have asked the question:
Is there a wall between The United States and Canada?
No, there is certainly nothing that would be understood as a wall along any significant part of the U.S.-Canada border. But keep in mind that the border is thousands of kilometers long, and it varies significantly as you move along it, and one thing that varies is how “fortified” the border is. The U.S.-Canadian border is frequently described as being the longest “undefended” border in the world, but that refers to the fact that the U.S. and Canadian militaries do not maintain installations along the border that are directed at each other. The law enforcement agencies of both countries certainly patrol the border very seriously, and crossing the border is serious business.
Like I said, it depends on where you are. The somewhat famous twin towns of Stanstead, Quebec, and Derby Line, Vermont, are basically a single town bisected by the border, where the border occasionally runs directly through buildings. Huge expanses of the border are uninhabited wilderness; over a thousand kilometers of the border between Alaska and Yukon is remote taiga and tundra. A significant part of the border is water – from the western end of Lake Superior through to the St. Lawrence River, the only way to cross the border is by bridge, tunnel, or boat, generally crossing waterways that would be difficult to swim under even the best of conditions. These defined crossing points create natural choke points that make it easy to manage customs without need for any kind of wall. The busiest crossing on the entire border, between Detroit and Windsor, is alertly guarded on both sides by police who will question you pretty thoroughly on what your plans are.