Worshipper – “Conquest” [Campaign 2, Mission 3]

I also couldn't beat this mission on my first try. But then I quickly captured the two lower castles in the first two or three weeks, built up an army, and defeated Gilraen first, then the druids, and finally the last castle. And in general, after completing all six campaigns, I think the most difficult one is the one for the Sylvan faction!