I recently played a game of Caylus here and was confused with how one of the favor interactions worked. The end of the game was initiated by the last section of the Tower section in the castle being built. After this, royal favors were awarded. I received 3 favors because I had 6 houses in tower section of castle. For my first favor, I advanced on the building track to the final, Prestige Building, section. However I wasn't prompted to actually construct a prestige building. I had enough resources to build many of them, and also had two residences in place, but still was not prompted to build.
Am I missing an obvious rule here? If not, I'm not sure if this is a bug or if I somehow missed the prompt and just skipped past.