Skyhold is a "prison without walls".  While the normal human occupants are held in standard cells, the most powerful villians are held in check by the willpower of the Mentu, a high order of monks with powerful psionic abilities. 
 
It is here that Ryon del Soya was kept before being freed by Red Monika.  However, during their escape, a mana containment center was breached, causing an explosion that disrupted the concentration of the Mentu allowing many powerful criminals to escape.