How do i do that?

Destroy their castle and kill them before they respawn.

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i always kill the wizards first, then i destroy the castle.
at the beginning of a level i use fireball or if already available: meteor
in progressed level, i use meteor to kill and lightning lvl 3 to destroy the castle, it is the best way that i know.
does someone have other experience?

Well, since this topic has been resurrected:

The summon skeletal archers spell is one of the best ways to destroy an enemy castle.

Also, if you can get him to attack you inside a giffin flock, close to his castle, you can guarantee he will hit a griffin accidentally who will proceed to bring his castle to ruins extremely quickly.

Is there a way to do stuff before I get Zerg rushed by skeleton archers?

Actually defending against this is pretty easy. The skeleton archers die quickly and can't be taken out without much difficulty (e.g. castle growth or meteor). The other wizards don't use it too often. And for some reason often are not good at defending against it.

I think I'm on level 5 and was just finding it to be a frustrating mess. My understanding is that you can't do anything with mana until you have a castle, but trying to get a castle there was almost impossible before being taken out.

Even before you have a castle, the amount of mana you have determines how quickly it regenerates and how many spells you can cast.

For level 5, there is a settlement that can provide you with lots of mana to start with if you're having trouble defeating enemies. (you can possess buildings for mana in case you didn't know this already). Also, again in case you don't know, the castle spell kills many (though not all) enemies when it spawn/upgrades, so can be used as a strong offensive spell.

Yes, laying claim to mana does help even if you don't have your castle down yet. Iirc, most levels start you with just enough power to lay down a level 1 castle. Consider picking a site early and plopping it down pronto.

Also, enemy wizards will be gathering power while you are building up, yes. They are generally doing the same things you are doing: killing creatures and roaming around possessing mana. Usually they don't bother you early in the level.

After a while though, you'll almost always need to go pound down their castle (and usually eliminate them as well) to get the mana you need.

Most the wizards will be beaten by slowly chipping at them and draining their mana. It helps if you take advantage of their explorations to steal from them and wreck their castles when they can't respond to it quickly. Meanwhile you've got to make sure you are gathering up all the loose mana on the map as the AI will clean house if you don't.

Single best way to kill a wizard's castle is 3-4 Lightning Storm spells cast right on top of each other into center of castle. There is a 100% guarantee they will take a level 7 castle down to zero quickly and still be going so the wizard can't rebuild his castle on the same spot.

So tactic is to cast 3-4 Lighting Storm right on center of castle, and then when it hits level 1, cast several more at the wizard. Follow up with some meteors, but you probably won't need to.