Dryads in the Castle with a bonus on the first week — only in the case of a Dragon Utopia on the respawn. In most other cases — unicorns in the Stronghold. If you usually put unicorns in the Castle on the first week — our conversation is pointless because you play an order of magnitude stronger than I do.
Dragons on the second week for this Castle are a very good option, it seems to me. I would only seriously reconsider in one case: if I saw an external dwelling of unicorns behind guards and had a choice — put dragons or save up, capture the unicorn external dwelling and buy them out. But generally, dragons on the second week rule.