Quick rundown on spells and their effect in battle:
Heal - A truly essential spell, this allows you to give a health boost to any damaged troops who are standing within your area-of-effect circle.
Firestorm - As well as doing a fantastic amount of damage to multiple enemy targets, the Firestorm is especially useful against wooden obstacles.
Stun - If you're finding yourself overwhelmed by baddies in a level, consider heading back in for another attempt with this area-of-effect spell that temporarily freezes enemies caught within its radius.
Hammerstrike - This spell is to stone what the Firestorm is to wood. Use it against stone barricades, towers and keeps for maximum damage.
Bladestorm - Metal getting in the way? Throw the Bladestorm at it to chip away even the hardest of metal obstacles.
Shield - An extremely useful spell to have on hand is the Shield, which will increase the armor capability of any soldier standing in its casting range.