vampires - vampire lords
*get a stack of 25+ vampire lords and a powerful spell-caster and you're in pretty fantastic shape.
giants - titans
*sticks out because i don't even bother buying giants.
iron golems - steel golems
*a horde of steel golems can sustain a powerful spell-caster on its own, sometimes even through the end game. can't think of any other third level creature that can do that.
centaurs - centaur captains
*sticks out as being the best first level unit with decent attack and good speed for weak heroes looking to build up experience through exploration.
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