Possibly of Urartian origin, this 'Devil's Castle' was for centuries a Georgian stronghold known as Qajis Tsikhe. It remains one of northeast Turkey's most spectacular fortress ruins, standing on a rocky bluff that's framed on both sides by the plunging Karaçay Canyon. You can catch a distant glimpse from the Ardahan road, about 2km northwest of Çıldır Gölü, where the old and new roads divide.
To get a far closer view, descend from the old road through Yıldırımtepe hamlet and follow the signs on a 3km winding route of concrete-brick cobble paving. That lane's latter half is very narrow with minimal passing- or turning-places so you'd do better to walk it. At the time of research, the last 400m of access path to the ruins was closed for safety reasons, but views from outside are the main attraction anyway.