(CNN) It is a place haunted not so much by the ghosts of those imprisoned within, but by the words they left behind on the walls.

Deep inside the dusty bowels of Raqqa's stadium -- an infamous ISIS headquarters from where the terror group is said to have plotted attacks on the Western world -- are a series of makeshift cells.

Some, squat toilets, have been converted into solitary confinement or torture chambers. Yet one room stands out, with names and time served etched onto the white walls, where ISIS held those they suspected of betrayal or espionage.

On the lower level of the stadium, ISIS turned changing and locker rooms into prison cells, the Syrian Democratic Forces says.

Most eerie are a series of sentences, scrawled in English in weak ballpoint pen, just above one of the holes militants punched through the walls, allowing them to move between the rooms without using the more perilous corridors. The tiny manifesto reads:

"If you are reading this, there are four main reasons why you are here. 1. You did the crime and were caught red-handed. 2. Using Twitter GPS locations! Or leaving GPS locations upon switched upon "turned on" the mobile phone (sic). 3. Uploading videos and photos from a sensitive WIFI internet source, i.e. you need your Amir's permission which you didn't get. 4. A suspect! Off the street! The Police have a good reason to do this! Be patience! Be Patience! Be Patience! The enemy of the Muslims Satan will do every whispering while you stare at the wall or the floor."

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