All right, probably a few words are in order on how these spells work in the Zu campaign.
First, the spellcaster must roll a d20 to see if he can cast the spell properly. This is not Dying Earth were spells are clearly annotated, well-memorized pieces of verbal technology that work 100% the time. Some of the formulas are incomplete and every wizard has to finish them on his own. Some involve speaking in languages assuming a different vocal apparatus, so you just have to try and make it sound as close as you can. Sometimes the proper words change depending on the astrological constellations, so you have to keep that in mind, too
Anyway, you have to roll. D20 plus your INT modifier vs a target number - sometimes the circumstances might call for additional bonuses or penalties. If you and your spell are equal level, that number is 12 (so 50% chance of success, assuming high intelligence). If you're higher level than the spell, the target number goes down by 2 for each grade of difference. If you're lower level (a 2nd level wizard trying to cast a 3rd level spell), it goes up by the same. If you hit or exceed it, the spell goes off fine. If you miss, you've failed but you can try again. If you rolled an unmodified 1, you mangled it badly. Roll again: if this time is a success, the spell still failed but at least nothing bad happened to you. If it's a failure, you get the Critical Failure effects. If this second roll is also a 1, then it's a double failure and chances are it's been nice knowing your character. If you roll a natural 20 on your first roll, something good happens, depending on the spell. With the spells in the previous post, their effects gain a "bonus" level.
But that's not all; some spells are really powerful for their level, while the rest of the (human) world kind of fragile. Therefore, whenever you attempt to cast a level (successfully or otherwise), you lose 1 HP for every level of the spell. Since, as we know, HP doesn't only represent actual wounds but also luck, divine favour, "rolling with the blow" etc. etc. you've read your Gygax, it can just as well represent the extreme fatigue of spellcasting. Note that in order to compensate for this extra cost, spellcasters roll d6 for their HP, CON bonuses come into play at lower values then in, say, AD&D, and non-magical HP healing is a bit easier to acquire.
So roll to succeed AND HP cost. There's got to be some upside to all this, and indeed there is: no Vancian pre-memorisation. Cue grognards twitching in the mouth. Once you know a spell, you know that spell. Feel free to cast it as many times a day as you can afford, but then don't cry when a monster hits you after you've burned through your Hit Point reserves. (Plus, the more spells you cast from those general reserves of yours, the more die will come up as a 1.)
And of course, a significant part of this is as yet completely untested (under real gaming circumstances, solo simulations are a different matter). Actually running this is the next thing down on the list, though, so it should happen in a few weeks. I did promise you "mad science" vibes, after all. If you don't know what sort of testing and peer reviewing procedures that means, you have to read Girl Genius.
One reason for instituting this system was a desire to try and solve one of the most commonly cited grievances with 1st level wizards: you cast your Sleep / Magic Missile and you're out of the game until tomorrow morning. This way, low-level wizards can cast a significantly larger number of low-level spells (with some associated risks), while high-level ones won't be able to just douse the battlefield in magic: even with d6 Hit Dice, they (like all other classes) will cap out at 5th level, so they will have to be conservative with those high level spells that cost 4 or 5 HP a pop. And if they decide to go full hog on those nevertheless, then they won't have an untapped standby reserve of a dozen low-level spells. So I'm hoping these rules will flatten out the wizard's "Suck at low level, rule supreme at high" power curve.