This is camouflaged very well. How long did it take to find it? I'm assuming not very long because it looks like you still had plenty of daylight left.
I'm left wondering about the arches. They could have used glass, they had the money. But a more practical approach, if they did need to fill the arches, would have been wood. No?
I am not sure glass was about at the time of design, but what else would they fill those arches with? It must have been very dark inside old castles before the 14th century. Lots of lit torches and smoke.