Since the ancient Romans erected their empire-defining stone walls, governments have continually built walls to keep out invaders, crusaders, foreigners — and to control their own populations.

During the Cold War, the Berlin Wall became the most notorious wall in history designed to keep citizens inside its boundaries. Yet the most iconic photos of the Berlin Wall are from the eve of its fall: November 9, 1989, when East Germans defied their captors and breached the wall.

Do physical walls work as instruments of border control in the 21st century? Do such physical deterrents offer a humane solution to the global issue of migration?

Since the Berlin Wall fell in 1989, more than 40 countries around the world have erected border walls. From Israel's West Bank Wall to new walls (mostly fences) currently being erected in Eastern Europe, these are some of the most contested border-control projects in the world.

—By Sarah Chandler, special to CNBC.com
Posted 09 October 2015

On Sept. 15, it was finished: a razor wire fence erected along the Hungary's southern border with Serbia. This was no flashback to the Balkan conflict of the 1990s, during which time landmines demarcated Serbian borders. It was simply another defensive — or, to many critics, offensive –– move by Hungarian Prime Minister Viktor Orbán's government, which has drawn international condemnation for its anti-immigration policies. Within hours the fence was tested: Handfuls of people were arrested trying to climb it.

To Dr. Alexandru Balas, director of the Clark Center for International Education at SUNY Cortland, history shows that physical border defenses don't work, and aren't humane. "Walls, from the Great Wall of China to Hadrian's Wall, have not kept people from trying and succeeding to get over them," Balas said. "Even if Hungary would electrify the wall, desperate people would find a way to get over it, risking their lives in the process." He added that there is a sad irony at work: "Some of the people suggesting this solution are themselves second-, third- or fourth-generation immigrants."

The wall along Greece's eastern border with Turkey was actually erected several years before images of refugees from the Middle East made headlines. Between 2011 and 2013, the European border agency Frontex sunk 37 million euros into Operation Poseidon. As more migrants risk dangerous sea crossings from Turkey's coast to the Greek islands, the operation's moniker has taken on a bleak meaning: a wall that reroutes refugees toward the rough, increasingly deadly waters of the Aegean Sea.

"People always have a way of circumventing a wall," said Dr. Ibrahim Awad, director of the American University in Cairo's Center for Migration and Refugee Studies. "But that keeps getting more costly, financially and in terms of human lives and human suffering."

More control of international waters is not necessarily the solution. "When the EU was more or less effective in patrolling the Mediterranean, they (the refugees) went to land routes," Awad said. "Just as water always finds a way of flowing to other channels."

The English Channel has long insulated Britain from the threat of invaders. Now it is one of the prime battlegrounds over U.K. security centers on the French port of Calais. In recent months, migrants have used various methods to cross, often breaking into trucks to make the passage through the Channel Tunnel as stowaways. In the past year, border authorities stopped people from making illegal crossings 39,000 times. A new high-security fence will stretch more than 2 miles.

French and British authorities are also trying to control the growing encampments in Calais itself, as desperate refugees erect temporary shelter near ferry terminals and tunnel entrances as they await crossing. The poor conditions in these camps have drawn widespread concern.

Protests and sit-ins have arisen as police continue to dismantle the chaotic camps, firing teargas at crowds. The EU announced last month that it would spend €5 million in humanitarian aid to fund a Calais refugee center, and relocate refugees to other parts of France.

During the Cold War, this border was among the most heavily fortified in Europe. And one of the most dangerous, with two fences on either side of a minefield that functioned as a 500-meter no-mans-land between the Iron Curtain and NATO countries. A member of NATO, Turkey was at the time part of the ideological West –– though geographically east of Bulgaria and the other Warsaw Pact countries.

In the late 1990s, the wall was painstakingly dismantled and de-mined. Bulgaria cleared more than 17,000 mines by 1999.

Last September the first 20-mile section of a new razor-wire border fence along Turkey's northern border with Bulgaria reached completion. Monitored day and night by Bulgarian guards and police, the imposing 15-foot-high fence now stretches 50 miles. And while reports of illegal crossings at this border decreased significantly from 2013 to 2014, the number of migrants entering Bulgaria has continued to grow –– indicating they are merely finding other points of entry.

According to the Bulgarian State Agency for Refugees, the agency received less than 1,400 asylum applications in 2012; by last year that number had reached 11,081.

Jerusalem's 16th-century walls — which date back to the reign of Sultan Suleiman I — were originally built to keep invaders out of Old City, then part of the Ottoman Empire. Now they serve as a beloved UNESCO Heritage site, visited by 3.5 million tourists a year.

Other walls aren't tourist attractions. Since 1994, an Israeli wall surrounds Palestinians living in the Gaza Strip. In 2004, in response to years of terrorist attacks, Israel began constructing the West Bank Wall largely on Palestinian territory.

The Hague's International Court of Justice has debated the wall's legality. Aziz Abu Sarah, executive director at George Mason University's Center for World Religions, Diplomacy and Conflict Resolution, said debate is further fueled by the fact that "there were no negotiations about the wall and no international group to monitor it."

The wall functions as a metaphor for the intractable Palestinian-Israeli conflict. "Israelis and Palestinians today are divided by invisible walls," Abu Sarah said. "The [West Bank] wall creates a situation where Palestinians and Israelis cannot meet anymore. ... The lack of interaction creates a fertile ground for radicalization on both sides."

Billionaire and now President Donald Trump drew cheers when he declared "Walls work" during his September 2016 pre-GOP debate rally, delivered in Dallas to a throng of supporters. Employing the signature rhetoric criticized by political commentators — heavy on incendiary catchphrases and light on policy — the subject of walls became a focal point of his talk on the way to election as the 45th president of the U.S. In Trump's case, the wall over which he obsesses building is along the 2,000-mile United States-Mexico border.

"Some politicians said Trump will never be able to afford the wall," Trump said at the rally. "It's peanuts. It's peanuts. It's nothing. And Mexico is going to pay."

Now Trump is following through on that promise, and escalating tensions with Mexico. Senate leader Mitch McConnell said it will follow the president's order, and he estimated it will cost $12 billion to $15 billion (estimates from the Government Accountability Office that pre-date the Trump administration suggest a price tag as high as $25 billion.) Trump's claim that it will be "easy" to erect and pay for such a wall remains a bold one. Currently, between one-quarter and one-third of the border –– about 653 miles –– is divided by fencing, which has already cost taxpayers billions. That leaves another 1,347 miles of fencing to build at a cost estimated to run as high as $9 million per mile to $16 million per mile.

According to many experts, the scenic rivers, mountains, deserts and ranch lands of the Southwest –– celebrated for both its beauty and its ruggedness –– would make building the kind of physical wall that Trump envisions an engineering and logistical nightmare. Or, in other words, very expensive.