In the centre of Raqqa is the main football stadium.
They are holding victory celebrations here at the moment but throughout Islamic State's reign of terror it was synonymous with death.
People brought here rarely left.
A group of soldiers told me to follow them over rubble into the bowels of the building to understand the sheer horror of the place.
The soldiers said it was the prison of death as I scrambled down into a long dark corridor.
They said the first section was for women. Further down it was for men.
This was a vast prison complex they said could hold between 500 and even thousands at a time.
The prison cells are of different sizes - some large to accommodate groups of prisoners, others tiny breeze block chambers a few metres square.
One was a tiny box for extreme punishment.
Discovered just days ago, the prison felt dreadfully unsafe as we worked our way through it. The soldiers told me to keep to hard surfaces and avoid anything in a sack lying on the floor.
In some of the cells the condemned had written messages to their families on the walls.
A final goodbye.
One large cell was reserved for Islamic State fighters. The soldiers told me that any of the fighters who had tried to desert or had expressed a desire to leave ended up here.
Desertion: a capital offence.
There were the names of men from many countries but also scratched on the wall was Abu Sa'eed Britani.
The date: 24 February 2016, scratches on the wall indicating how long he had been held.
If it was him, he was also known as Omar Hussain, who formerly worked at a Morrisons supermarket as a security guard and travelled to Syria to join IS in 2014.
This may have been where he died.
Was this where Briton Omar Hussain - aka Abu Sa'eed Britani - was held?
As the prisoners were interrogated and processed they were moved along the corridor - the real horror about to begin.
In one chamber, two metal beds still remain. It is where the prisoners were strapped down and electrocuted.
Worse was still to come.
Next door they were tied to weight machines that, when lifted, bent their bodies backwards, perhaps breaking their backs.
In an anti-room they were hung on hooks and flogged.
Dazed and in agony, they were dragged out and upstairs into daylight and the football stadium.
There they were executed.
This is the worst place I have ever been and Islamic State's most gruesome prison.
