Warlock – “Cultist” [Campaign 4, Mission 3]

...In my opinion, you just need to capture at least 5 castles, and then everything will be easy...

True, capturing the first five castles will require some effort :)
In principle, everything has already been said; this mission is really very difficult at the beginning (until you capture five castles), and then it's just a matter of technique (capturing the sixth and seventh)...