Two years ago, that border comprised two parallel dirt tracks, one on the Bulgarian, one on the Turkish side.
Now a razor-wire fence, 1.5m (5ft) thick, welcomes would-be migrants. Thirty kilometres is already completed, while 100km more remains under construction.
"The purpose of the fence," says Philip Gunev, deputy interior minister, "is to divert flows of migrants towards border crossing points where our limited financial resources allow us to protect European borders in a more efficient way".
Such official checkpoints, he insists, are safer for asylum seekers than trudging long distances, often with small children, over the rough, hilly terrain the fence now cuts across.
Fixed and mobile cameras, mounted on four-by-four vehicles, complete the picture along the whole length of the border.
In the past eight years, since joining the European Union, Bulgaria has spent €300m (£215m) of mostly EU money, reinforcing this border. Another €100m is available to complete the job until 2020. Only €2m will be received for the better integration of refugees in the same period.
Krassimir Kanev, director of the Bulgarian Helsinki Committee, a human rights advocacy groups, is unhappy with the checkpoints.
"The only way to get through is to pay smugglers," he says, arguing that only the richer migrants get a chance to try. "And there's nothing safe about being cramped in the hidden compartment of a truck in the heat of summer."
Refugees in Bulgaria
11,081 asylum seekers in 2014
6,000 asylum seekers this year, so far
Bulgaria granted 5,162 people refugee status in 2014 - right to reside, to work, to healthcare and welfare
Another 1,838 were granted humanitarian status - limited rights usually for a year
About one third of Bulgaria's migrants are caught on the border with Turkey, another third as they head north or west inside Bulgaria, and the rest on the Serbian or Romanian borders, as they try to continue their journey towards Hungary on their way to Germany.
Bulgaria has one of the highest rates of granting refugee status in the EU. Refugee status means they also receive a Convention Travel Document (CTD) and under the 1951 Refugee Convention they can travel on to anywhere in the EU and stay for up to 90 days.
In practice, few ever come back, travelling to Germany or elsewhere.
At the Harmanli refugee camp in south-eastern Bulgaria, two police buses bring more asylum seekers.
Children wave happily, adults look more concerned.
Conditions here are much better than they were in 2013, when overcrowding, appalling sanitary conditions, and the alleged cruelty of guards gave Bulgaria a bad name.
Some asylum seekers still express frustration at delays with their applications.
A group of men hold up a snake they killed in the camp the day before. But most acknowledge a big improvement in conditions for the 1,600 refugees here.
Bulgaria is facing growing pressure from Western governments to identify exactly who they do let in.
Ninety percent arrive with no documents whatsoever because they were taken by the smugglers who brought them this far.
In an upstairs room at Harmanli, officers from the Bulgarian intelligence services cross-examine the refugees, most of whom are Syrian Kurds.
While Harmanli is an open camp, those deemed suspect are taken to a prison at Busmantsi, near Sofia, where they can be detained for up to a year, while more investigations are carried out.
"The most frustrating thing about life there was the waiting," said one former Busmantsi inmate, who asked not to be named.
"Your whole life is waiting. You know there will be an end to all this, and one day you will be out, but at this moment you have nothing to do but wait."
Are there radical Islamists inside the prison? I ask.
"People keep themselves to themselves. They only share what they have to," he tells me. "But the radical mood among my friends is all about money, which comes mostly from Saudi Arabia. It has nothing to do with political or religious beliefs."
"Don't link those fleeing terror with those who would like to create it," says Boris Cheshirkov, a UN refugee agency spokesman in Bulgaria. "States can protect refugees, and address security concerns too, by screening and registering them early on."