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That's true. The Stronghold is indeed the most challenging. It doesn't provide crystals, and you need a lot of them for Cyclops and Ogres. Also, it usually turns out that Cyclops are built last (and in the swamp, Wyverns are built early). In general, each castle is unique in its own way. And I repeat: whoever is best at playing with a particular castle will find that castle to be the strongest for them. Although, it's best to develop a game strategy for all castles.