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I don't understand the people who voted Life as the worst faction. It has simply excellent heroes that emerge from the Seminary Castle. And their magic is a perfect match. Life magic is the best support for a warrior hero (magic resistance makes it impossible to affect the opponent's hero with curses, but a good blessing can easily give even a Barbarian a hard time), and Order is the strongest school in the game, and doesn't need any further explanation. Of course, the troops don't bring much joy, but they're not completely weak either. And with the support of Tactics, Life, and Order, they become quite decent. The Priest eventually becomes a Paladin and goes to war alone, like a Barbarian (and he actually does it even better). Meanwhile, the Knight learns Order magic and remains with his troops, strengthening them and helping in battle with control spells.
If I had to choose an underdog, I would name Strength (and this is paradoxical). Its troops are only noticeably better than the competition at the beginning, and you have to look for magic elsewhere. Its only real strength is the Barbarian hero, which is unique to this Castle. Indeed, 3 skills instead of two and the starting melee skill are pleasing, but the Nature Ranger, who also has Warfare, is only slightly behind, but the Castle itself offers many more opportunities. Strength is a game about blitzkrieg, which can be ruined by an inconvenient group of neutral creatures standing in the wrong place. And the further Strength gets from the beginning of the game, the more its lag behind other Castles increases.