Thank you so much! Yes, I really wanted to make a scenario that screws the PCs no matter what they do, in an interesting way, of course!
Yes, the Giant Snake is bolded, which calls it out as a creature found in the Shadowdark core rulebook. I was going to append a page number, but I was in a tug o' war with economy of space, so I left it out.
The DC to open the door is high, but since the guards have been alerted, they will soon be coming through it to apprehend the PCs. Also, the open-air nature of the cloister begs them to escape by climbing out. A treacherous climax indeed! As Valeria said to Conan, “Do you want to live forever?!”