Having a strong defense is just as important as having a strong offense. Defenses serve to safeguard trophies and protect resources from enemy troops. Each defense has its own strengths and weaknesses, and its location should reflect that. For instance, the powerful Mortar deals incredible splash damage to ground troops, but cannot hit enemy troops within four tiles of itself. So it should be placed toward the center of your base to allow it to be the most effective. On the other hand, cannons and watch towers have a higher dps than that of the mortar and will be more effective against the tankier front line of the enemy army ie. gunners and iron storms. Therefore, they should be placed toward the outside of the base.
