I think one thing that made this especially interesting to me was the nature of the "magical guardians" - each key is guarded by a sorcerer who basically obtained Ultimate Magical Knowledge, and the key was given them as a reward, and grants them immortality. If they give up the key, they start to age and die. So not only can you sort of see where persuading each sorcerer to give up their key is inherently going to be hard, there was this psychological dimension of "what did this sorcerer think they wanted vs. what they actually NEED." (I don't think it's a spoiler to say that the sorcerers all have Immense Emotional Baggage.)

Possibly also relevant: the author, Geraldine Harris, is an Egyptologist at Oxford.