Interesting history of the security of walls:
Dún Aonghasa presents early evidence of the same principles of redundant security measures at work in 13th century castles, 17th century star-shaped artillery fortifications, and even "defense in depth" security architecture promoted today by the National Institute of Standards and Technology, the Nuclear Regulatory Commission, and countless other security organizations world-wide.
Security advances throughout the centuries have been mostly technical adjustments in response to evolving weaponry. Fortification -- the art and science of protecting a place by imposing a barrier between you and an enemy -- is as ancient as humanity. From the standpoint of theory, however, there is very little about modern network or airport security that could not be learned from a 17th century artillery manual. That should trouble us more than it does.
Fortification depends on walls as a demarcation between attacker and defender. The very first priority action listed in the 2017 National Security Strategy states: "We will secure our borders through the construction of a border wall, the use of multilayered defenses and advanced technology, the employment of additional personnel, and other measures." The National Security Strategy, as well as the executive order just preceding it, are just formal language to describe the recurrent and popular idea of a grand border wall as a central tool of strategic security. There's been a lot said about the costs of the wall. But, as the American finger hovers over the Hadrian's Wall 2.0 button, whether or not a wall will actually improve national security depends a lot on how walls work, but moreso, how they fail.
Lots more at the link.