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This is actually an interesting question – is it better to focus on destroying the walls to pass through on foot over the corpses, or to destroy the creatures behind the walls (with ranged attacks and magic, luring the besieged enemy out)?
By the way, based on my (somewhat limited) experience, I can advise you to never forget to fill ALL the skill slots for your hero until you have built 5 levels of the Mage Guild and explored all the available territory.
Usually, I think something like this: at each level of magic, there are key spells for me that determine my further development and tactics:
1st level - Slow, Haste (if available - requires corresponding magic)
2nd level - Clairvoyance (Clairvoyance is indispensable for diplomacy)
3rd level - Earthquake (if available + earth magic - a ballista is not needed)
4th level - there are many spells here - Prayer, Clone, Portal, Berserk, Armageddon (if available - you need to level up the Resistance skill)
5th level - Dimension Door, Fly, Explosion (if available - air or earth magic is REQUIRED)
Of course, I'm exaggerating a lot; you can't write everything in a forum post - it's better to discuss these things over a beer (sorry for the off-topic)