A THREAD FOR NEWCOMERS!

Does the "Intelligence" skill and the Hero's "Knowledge" stat both increase the maximum mana pool? Also, it's not entirely clear what "maximum spell points" means.

1 point of Knowledge grants 10 mana. Intelligence increases mana based on this. For example, with 12 Knowledge and Expert Intelligence, you'll have 12*10*2=240 mana.

Then, does the "Magic" skill increase spell power, i.e., enhance the Hero's "Magic" stat? And does this, in turn, increase the power of spells?

Magic increases the damage dealt by impact magic. For example, if Lightning deals 125 damage, with Expert Magic, it will deal 125*1.15=143 damage.

Is it worthwhile to invest in Catapult? I once leveled it up, and it destroys walls in 1 or 2 hits, but it's not really useful because I don't go into the city anyway. The opponent either comes out on their own, or I destroy them from a distance.

Catapult is a useless skill; there are many other skills that are much more useful. Of course, there are situations where it can be genuinely helpful. For example, when the opponent is holed up in a castle and doesn't want to come out. Controlling the catapult also allows you to use magic during its turn (before the units' turn)