Very solid visual design and gameability, this can be dropped in basically any setting with very little effort. The theme is nicely played and I like the idea of this being both an investigation and a rescue mission.
It gets a bit weak on the actual citadel, though. The PCs are expected to be discreet but there’s no reason for them to: the citadel is full of still-loyal citizens (some of them armed guards) and the cultists cannot interrupt their ritual, not even to kick the PCs, without losing. It would have been better if the citadel was controlled (or reportedly controlled) by cultists, giving the PCs a good reason to fear discovery.