The photo was submitted to the 'ghost' photograph project, the project received around 250 ghostly submissions and the public was invited to comment and vote on what photos they thought were the bee's knees, the castle photo took top spot. You can view the top 10 most voted photos at the link.
One explanation that's being discussed is that the face is actually light shinning on rocks behind the bars of the window and I can really see that as a possibility, good old Pareidolia.
Here's a video about this photo and others that were submitted to the project. The dude reporting is at the castle, it would have been nice if he actually checked out the window where the ghost was photographed, I'm so wondering if there's another window in there or a hole even.