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In my opinion, the strongest castle is Order.
1. Hobbits deal bonus damage to 4th-level warriors.
2. Dwarves have magic resistance.
3. Golems are armor with 75% magic resistance.
4. Mages are very weak, but the magic is decent (Poison, for example. It will get any hero :) )
5. Djinns have good spells. (Create Illusion, Ice Arrow...)
6. Nagas provide no-retaliation. But... nagas have low growth (4 per week) and are slow.
7. Titans are shooters who receive no penalty in melee combat + Chaos Ward.
8. Dragon Golem is a fast unit with first strike. Powerful. It is a construct, so it has resistance to magic affecting living creatures (Song of Peace...).
The hero pumps Nobility and along with it the treasurer, miner, etc. Economy and bonus creature growth are present. Order magic, in my view, has everything. Mind magic (Hypnosis, Berserk), attacking (Ice Arrow), defense (Dispel Magic), curses (Slow, Forgetfulness), blessings (Haze, Accuracy), even "stealing" (Steal Spell or All Spells).
Conclusion: Order rules :)