Exploration of any ruined structure can be incredibly dangerous, and every care should be taken to ensure your safety.
There were no ‘no trespassing’ signs posted outside of Merlin Castle, and the door had already been forced open by local teenagers, so I assume that my crime was, at most, unauthorized access.
Given how stoutly Merlin Castle is usually locked and barred, the open door was a wardrobe-to-Narnia invitation. The interior of the fortified house is intact, to the point that I could navigate the interior stairs up onto the roof of the castle, explore most of the rooms and cells and even make it into the cellar below the main floor. A combined lack of either torch or flashgun and the time with which to explore prevented more photographs, although now I really want to get back out to Merlin Woods and into the castle again before I leave for American on Wednesday.