The fortress occupied by King Valerin, and possibly a kingdom.
Valerin kidnapped Guinevere and brought her back to the Tangled Wood. The fortress – which is reminiscent of an otherworldly location – was set high upon a mountain, and was cordoned by a tangle of brambles, branches, and snakes that kept out everyone.
Arthur could not break the defenses, and only an intervention by the grand wizard Malduc could undo the magic wall.
Source
Lanzelet | Ulrich von Zatzikhoven, c. 1200