ZFR: Even before you have a castle, the amount of mana you have determines how quickly it regenerates and 
 ShadowWulfe: Hm, but the longer you take to kill off monsters the more time the opponent wizard has to build his stuff. I guess the real problem I'm having is that my spells are so weaksauce right now.  
 But laying claim to mana still counts before you even build a base. 
 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.