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)