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The Citadel simply has no other option but to invest as much as possible in its army, because many consider this castle to be one of the weakest. By day 8, you definitely need bears, griffins, and therefore goblins and birds. And you need to buy all of this; the castle's income is definitely not enough, so you need to find alternative ways to finance it. In the case of Necropolis, the situation is completely different; it focuses on lords, more heroes with special necromancy skills, and conserving resources.
Of course, you can design many maps that require a completely different tactic; they are available in the standard set, but for a very wide range of templates, Gynnar's reasoning is more than valid.
However, often the game is played for relaxation, and in that case, you can ignore many things.