Game Mechanics and Glitches - Some explanations about how the game works, specific techniques, and glitches

Main strategies

Castle is the best spell in the game for killing most, but not all monsters. However, there are numerous situation where you need to build up your castle. You can raze your castle again by pressing Shift+L.

Sufficient mana must be gathered before starting mana gathering missions (see bug note). Planning on which mana to get and optimizing balloon routes (at least on the earlier levels) is crucial. (And no, there is no way to control your balloons.)

When the mission objective is to kill all monsters of a particular kind, it usually cares about all monsters of that kind that are alive at the moment. There are often ways to spawn new monster of that kind, which will then be included in the objective, so try to avoid that as much as possible.