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If you build a Citadel and a Castle, this is most effective for the Stronghold faction, and the additional 6000 resources are better spent on more useful buildings – an Arsenal and a Sanctuary. If you're playing as the Griffin faction, you can save a day, but it's still more expensive overall. This is definitely true – even with what seem like the simplest buildings, upgrading them to level 4 is one of the most expensive things to do.