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You still have to reach the enemy castle... And what if you encounter a thief hero?
Then ballistas are a one-way ticket to defeat.
But they win in battle. Pikemen are not bad, I agree, but they have too many drawbacks. Their slowness on the battlefield (imagine if you also cast a slowing spell) and they always suffer more losses (a favorite target along with squires).
Zdob, respect! Excellent castle for comparison.