Trump's wall

Donald Trump, Phoenix, AZ, 30 August 2016:

We will build a great wall along the southern border... On day one, we will begin working on intangible, physical, tall, power, beautiful southern border wall...

Let's take a look at some other walls designed to keep folks out.

Hadrian's Wall was a defensive fortification in the Roman province of Britannia, begun in 122 AD in the reign of the emperor Hadrian. It ran from the banks of the River Tyne near the North Sea to the Solway Firth on the Irish Sea, and was the northern limit of the Roman Empire. ...

The limites of Rome were never expected to stop tribes from migrating or armies from invading, and while a frontier protected by a palisade or stone wall would help curb cattle-raiders and the incursions of other small groups, the economic viability of constructing and keeping guarded a wall 72 miles (116 km) long along a sparsely populated border to stop small-scale raiding is dubious.

Didn't work all that well.
But wait!

Great Wall of China is a series of fortifications made of stone, brick, tamped earth, wood, and other materials, generally built along an east-to-west line across the historical northern borders of China to protect the Chinese states and empires against the raids and invasions of the various nomadic groups of the Eurasian Steppe.

Just a reminder: When Marco Polo visited China, the guy in charge was Kublai Khan - a Mongol... one of the folks the Great Wall was designed to keep out.

... and here's my favorite:

The Maginot Line was a line of concrete fortifications, obstacles and weapon installations that France constructed on the French side of its borders with Switzerland, Germany and Luxembourg during the 1930s.

For what it's worth: the Maginot Line worked like a charm - no one ever got through it... The Wehrmacht just went around!

There is no such thing as an impenetrable border... but it costs a LOT of $$$ to try to build one.
... And what's the point?