How should Defense spells be implemented?

Note:Please choose one number and one letter based on the idea you like.

Supercell has been considering the idea of defensive spells for a while now. How would you like this implemented?
Ive come up with a few ideas.

Structure:
1. A defensive structure that shoots "mini-spells".
2. A spell trap.
3. A structure that weakens the effects of other spells.

Spells:
a. A building that converts offensive spells into new defensive ones(Lightning spell turns into Fire spell).
b. New spell factory and spells altogether.
c. Keep the old spells and use them defensivley(ex: heal spell heals buildings, jump spell levels up walls,etc.)

I'll develop an idea once poll is conclusive.