The Grey Wardens' prison in the Vimmark Mountains is believed to have been constructed more than a thousand years ago. The original method of construction has been lost to history, but the Warden-Commanders of the Free Marches have maintained the prison's secret through the centuries.
The prison is concealed in a great rift in the Vimmark Mountains, far from any easily-traveled mountain passes. The Wardens themselves have spread rumors of banditry and beasts to prevent explorers from approaching.
The prison consists of a central tower built into the rift with magically-maintained bridges allowing access at different levels. Each level is sealed by a blood magic ritual in which a mage of untainted blood uses his own life essence to create a magical barrier that is permeable from the outside yet impenetrable from within. This one-way access has caused other darkspawn—and perhaps unwary travelers—to be caught within the prison's confines. Those who disappear inside never re-emerge.