(and ballistics, in my opinion, is one of the most useless skills)

Nonsense. With an expert in ballistics, you can quickly destroy the castle gate (the only path not through spikes for ground units), and besides, not much will remain of the archers. Otherwise, the army will simply crowd near the castle gate, without being able to enter. And cyclops kings make 2 attacks on the walls, so if you have, say, few cyclops, it would be wiser to attack the castle walls and allow other units to clear the area, rather than kill a couple of monsters.