As you already know, there are two types of spells: offensive, which affects the opponent directly, and defensive, which affects only the champion, his owner, or friendly cards. Champions can be immune to some offensive spells, but only a few can be immune to defensive spells.
Spells can be cast only during the phases listed on the card during your turn. Some spells can be played at any time, and this will be noted on the card. In addition, all spells must have a target. For example, you can't Dispel Magic if there is no spell or effect to dispel.
If card play prevents an attacking champion from continuing to fight, that champion is defeated and returns to its pool, but the battle may continue with another champion. If there is no other champion, the battle is over. Since the champion was not discarded, the defender does not get the spoils of victory.
An attacking champion always has the opportunity to play a single card that will enable him to continue the battle (regardless of current combat totals), such as the Fly spell.
If card play prevents a defending champion from continuing to fight, then combat is over. The defending champion returns to the player's pool. The defending player razes the realm, and the attacking player may draw the spoils.
Dispel Magic, Spell Turning, and Reflection
You may cast these kinds of spells in response whenever another spell is cast. Spell Turning, Reflection, Redirect work only on offensive spells. Depending on a phase, the spell's effect may be redirected back at the caster or to other champions "in play."
The Wish spell is one of the most powerful spells in the game. It ignores all cards that state they are immune or otherwise unaffected by spells unless they state immunity to the Wish spell. Regardless of a champion's immunities or special powers, Wish can discard them (unless a champion's special power states that it is immune to the Wish spell itself).