Which units would you replace in the Sylvan castle?

Remove the green dragons and replace them with Ents (slightly increase their stats, and everything will be fine).
Level 6 - Druids (with more health and attack).
Level 4 - the Tigers from the fourth game, which were unfairly forgotten. Or some other kind of beast.

Yes, I agree that the tigers were unfairly forgotten; they were strong units in the fourth game. As for the dragons, I don't really care, as I don't play as that faction much. However, putting Ents on level 7 would be quite logical, as this is a nature castle. I think level 5 is enough for the Druids; level 6 seems a bit too high for them. On level