Discovering Merlin's Castle

I have found the location of Merlin's castle on an ancient medieval map created by a a Benedictine monk in the 13th Century. Unfortunately, while I was busy packing up my bullwhip, fedora, satchel and leather jacket, I remembered that my discovery wasn't exactly new. My search for the Holy Grail has therefore been postponed once again.

The Parker Library at Corpus Christi College, Cambridge is