Meh. The alchemy interactions are an interesting mechanic, but the rest isn't very fun. You stop being able to purchase units because you "need more tunnel space" but there doesn't seem to be any way to get more tunnel space or to sell / retrain units, so you're stuck with whatever set of units you've already bought when you find this out, or hand-clicking for an absurdly long time on replay to purchase more optimal higher-level units.