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Yes, the ballista is not such a useless unit at the beginning of the game. The ballista deals damage equal to (2-3)*(Hero Strength + 1).
Let's say we have a Barbarian hero (not Gurnisson!). Then, without any upgrades, his ballista will deal
(2-3)*(4+1) = 10-15 damage.
I agree, it's not much, but it replaces about 4-3 extra Orcs.
Next, let's just look at how much damage it will deal as the Strength skill is upgraded:
5: Ballista damage - 12-18
6: Ballista damage - 14-21
7: Ballista damage - 16-24
As you can see, you can achieve quite good results already in the first week. For example (I once calculated), the damage dealt by an army, using basic creatures of levels 1-4, is on average about 70-150 damage units. If you add the ballista's damage to this, the benefit will be obvious.
But there is one "BUT" - at the beginning of the game, the ballista is too expensive a unit, and, frankly, I would prefer to buy, for example, 3 Rocs instead. Cheaper, more practical, and more powerful. :)
It's a different story when you have a hero who specializes in ballistas. My favorite is Gurnisson, who already has a free ballista and the Artillery skill, which now needs to be perfected. Without going into details about how much damage the ballista deals at levels 1 and 2 of the Artillery skill (I don't like probabilities), let's immediately consider the Expert damage, which Gurnisson will be able to achieve very easily:
(2-3)*(Strength + 1)*(2 shots)*2
Damage Dependence on Strength:
4: Damage - (40-60)
5: Damage - (48-72)
And so on.
All this, of course, does not take into account the distance, as well as the secondary skills Attack or Accuracy. Just keep in mind that if the enemy is within firing range without a penalty, the damage will be maximum.
And all this can be easily achieved in the first week. It's worth considering...
As for the Wisdom skill for a warrior, in my opinion, it is only useful at the end or in the middle of the game. In the early stages, you can easily do without light spells: Blessing, Curse, Shield, or Stone Skin.