I have a session coming up this weekend that'll very likely involve my players gaining entry into the fabled Iron Citadel. The Nexus of Power book has given me some good info and a fun encounter, but I was hoping the more seasoned world builders here could give me some ideas on obstacles in the Citadel itself, the layout out, and mainly just rooms and details that would make sense to be included for a fortress famed for never falling to attack. They'll be in for a hell of a fight with the BBEG for this chapter of the story, and in addition to the force witch encounter as well as other hijinks they're sure to find on their own, I'm trying not to make the Citadel too much of a slog for them, while at the same time making it memorable. Any tips or ideas would be greatly appreciated.