With US President-elect Donald Trump's promise to build a wall across the border with Mexico now a very real possibility, what can he learn from the other great walls in history?
Some of these walls still stand and do the job that was intended of them, but others have since been torn down, while some didn't work at all. (All images Getty)
Constructed by the Roman Empire almost 2000 years ago to protect modern-day England from raiding barbarian tribes in Scotland, this 120km-long stone structure still stands intact as the ultimate symbol of protectionism. A kind of ancient premonition of Brexit in reverse.
One of the Roman's most-expensive building projects in history, the wall successfully served its purpose for some 300 years.
So what can Mr Trump learn from Hadrian's Wall? You can successfully protect your borders if someone else is willing to pay for it.
Western Australia's great project to keep rabbits and other animals regarded as pests from its farming areas, the fence was actually one three that stretched over 3000 kilometres.
Built at the turn of last century, the wire fence was expensive to maintain and proved largely inadequate at keeping out the rabbits.
Mr Trump's lesson here? Build your wall high enough so it can't be jumped over.
The Maginot Line was a series of heavily armoured fortifications along the French border designed to protect France from a German invasion after World War I.
Built in the 1930s and thought impregnable, Hitler's forces simply went around the back door of the Maginot Line during World War II and successfully invaded France through Belgium and Luxembourg, where defences were far less built up.
What lesson can Mr Trump learn from France's great military blunder? Make sure your great wall goes all the way to the sea.
A 155km-long symbol of Soviet paranoia, this concrete barrier divided Germany's biggest city from 1961 until the Soviet Union itself came tumbling down in 1989.
It's thought around 5000 East Germans successfully got through the militarised blockade to freedom in the West, but more than 140 died trying.
What can Mr Trump learn from the Berlin's experiment? If people are desperate enough, they will find a way to get through any obstruction.
Perhaps the most famous wall ever built, this 21,000km monstrosity was built to protect China from northern invaders more than 2000 years ago. It served its purpose for centuries, and was eventually used not for defence, but migration control.
So what can Mr Trump learn from the Chinese? A wall can keep people out, but also let the ones you want in.
Built after the Korean War armistice in 1953 to provide a buffer zone between the North and South, the 250km DMZ is the most heavily militarised border in the world.
Despite plenty of bullets and bombs being fired across over the years (usually from the North), the DMZ continues to stop another bloody war from occurring on the Korean Peninsula.
What can Mr Trump learn from the DMZ? A well-constructed and funded wall can keep the peace, even with a power-hungry and paranoid megalomaniac on one side of it.