Fireball Catapult is a strong defense extremely effective against Tier 1 troops as it deals huge damage in a splash form. The best position for this defense is central in your base but as you begin to introduce, more of these other formation should be introduced. See Base Layouts when multiple of these are available to you.
On December 6th 2013, an upgrade was applied to Fireball Catapult that its damage was doubled (but attack speed was reduced to half). This changed the game a lot in offense and defense because from now on, Archers and Thieves will be all wiped out with only ONE hit from Fireball Catapult.