The European parliament will vote on whether Bulgaria and Romania can join the region's free travel zone, known as the Schengen agreement, this week.
The agreement allows people from member countries to travel without the need for any sort of visa.
As part of the process to gain Schengen support, Bulgaria has been making efforts to show its borders are sealed.
It has led to the creation of Bulgarian migrant detention centres, designed to look like a fortress.
As unwelcoming as it looks, it's similar to detention fortresses in most of western Europe.
People held in one fortress in Bulgaria were desperate to tell stories of their invisiblised lives.
In the first of our special series on European border controls, Al Jazeera's Laurence Lee reports from a Bulgarian detention centre.