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you forgot an important fact: if you have a controllable ballista (with a skill, that is), then (naturally, if the opponent doesn't have artillery, which gives control over the towers) the ballista moves first, and during its turn, you can cast a spell first! and this first cast can be critical!
all this is, of course, relevant if you are besieging a castle.
we're talking about a siege, right - in other cases, a ballista isn't that effective...
hmm... how can the first cast be critical? I don't see the point in casting mass slow... you won't destroy the entire army with the first cast...
Stay still, so I can kill