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Here’s another idea: use creatures from the new creature banks to exchange for desired spells. For example, for a level 5 spell, you give a reward from the maximum-level Crimson Tower, Factory, and Zephyr. For a level 4 spell, you give a reward from the pre-minimum level of those banks. It doesn't really matter which building this is tied to; in the end, you'll definitely get the spell you want, but you'll have to give up a good amount of creatures for it.