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Blue dragons are useful because they are the only creatures capable of inflicting Fear. In all my years playing Heroes III, I've only encountered this magic once in a guild... and even then, it seems it was in a campaign. Crystal dragons are useful for their crystal generation. Once you buy one, it's easier to get a second. Rusty dragons deal a lot of damage thanks to their acidic breath. I don't like fairy dragons. Among the neutral creatures, I love snipers. They are one of the best ranged units, perhaps even the best, although they have low health. Wizards shoot much more powerfully at close range. Peasants, as already mentioned, are only useful as skeletons, especially if they have joined your army. Then, a skeleton converter becomes useful. By the way, this is another factor that makes this castle strong.