Cannon Level 12 & Wall Level 11

 Chief, upgrade the Cannon to level 12 for awesome firepower and make your village perimeter near-impenetrable with level 11 Walls!!!

The Mortar is arguably the best defensive structure in the game. It's one of only two true splash damage defensive structures in the game and it's often the most crucial to protect. Intimidating and well placed Mortars have been known to keep attackers at bay.

Since the Mortar deals splash damage, it should be one of your earliest leveled structures. This structure will generally be the most effective deterent to lost resources so always put your gold into it before leveling other structures. Since defending from farmers usually consists of warding off large numbers of troops, Mortars quickly become invaluable.

Mortar placement is crucial. You want your mortars to be able to defend both the "core" area of your base as well as the outer walls. As mentioned before, well placed Mortars can keep enemies at bay (it's certainly kept me at bay) so make sure that they are one of your most fortified structures in your base (certainly the most fortified defensive structure).