Ok, I did it at last. Here's what I learned....

First possess the mana pool nearest you, kill the couple of birds and the kraken in the immediate vicintiy, and collect the rapid fireball spell on the taller mountain. Possess the closely grouped pools of mana near the short mountain, so you can keep track of all of it in your mini-map, and kill any vultures, wizards or krakens who invade this area. Use the heal spell after every fight. To kill the last wizard or two, it helps to manoeuvre during a duel so as to put some griffins between the two of you -- one of his fireballs inevitably goes astray, and then he's toast.

Mop up the krakens and vultures -- when all are destroyed, and all the free mana is in your possession, go and collect the lighting bolt spell from the tall mountain and go after the griffins. It will take a while to pick them all off one at a time, because they tend to group up -- but attacking a massed group is suicide. Then collect your well-earned castle spell.

You only need a size 4 or 5 castle to collect enough mana to restore the world, but after that nasty fight, I built a size 7 castle just so I could get all of it. It was very restful just to switch to the world screen and sit watching my balloons go out and suck it up ;-).