Author Jeffrey Glover explores symbolism carved by Templar prisoners in the dungeon of Donne castle in southwestern France in the March 2010 issue of Fortean Times.
From "Lost Graffiti of the Templars":
The assumption that first-hand evidence of the Templars’ mysteries was erased along with the structure of their organisation has been perpetuated by so many books that most researchers, scholars included, entirely ignore the fact that the Templars actually did leave behind some startling indications of their thoughts in the form of stone-carved graffiti in prisons where they were held following the suppression of 1307.
There are Templar graffiti in the dungeon at Warwick Castle in England and at Chinon Castle in France, but by far the strangest and most intriguing examples are to be found at the guardhouse at Domme, in south-western France – traces of the order that have been unaccountably overlooked in the thousands of pages written about the Templars. These wall carvings are as close to first-hand Templar writings are we are ever likely to get,  so when the opportunity arose to take a close look at them I seized my camera and sallied forth. Little did I suspect that what I was to find would leave me astonished and engulf most of my spare time in the following months as I became driven by the need to comprehend what the Templars had left behind on the walls of this terrible place.
Read the whole article here.