Turns out that the walls to the castle are too difficult to scale undetected so we have taken a second, deeper route. The castle is somehow built over an abyss full of choking poisonous gas. How charming, but it provides us an alternate path, one full of crates and guards with Night Vision.
It is interesting how ordinary guards with night vision turn out to be, to me they are effectively just guards that are always using their torches and actually the bigger obstacle they present is their immunity to the toxic gas found in my upgraded smoke bombs.
The final encounter in this level is particularly interesting, it is an escalating survive for at least 60 seconds scenario and one I am not entirely sure how I would have handled without the Mask of Nightmares. Do the guards raise the alarm during this encounter? I wouldn't know because for me they simply panicked whenever they saw a corpse. I love that mask.