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The Berserk, which I bought in the spell library, helped in destroying two strong Knights around the second Castle. Of course, I didn't give away the Ring of Speed. I passed the level quite quickly – I just explored the area nearby, collected upgrades, and moved on. 170+ Monks and a super unit solved the problem of destroying the Chaos Knight.
But without training buildings, it would be a problem to gather an army to destroy three Knights, especially after the first Castle, where there are many losses.
I think that if you create a campaign with a low level limit, taking the first Castle without the ability to buy a Ballista is somewhat problematic (although I took it using only swords; maybe I should have tried using summons as an option, or taken the Castle earlier. But there were also problems with breaking into the library). It's a good map.