Cannons are the least expensive of all of the defenses to upgrade. They have a range of 9 tiles which is 1 less than their counterpart the Archer Tower. This long range makes them a valuable part of your defenses. They do damage to a single target and once locked onto that target they can not select a different target until the 1st target is either destroyed or moves out of the Cannons range. Cannons are vital in the defense against the larger tanking troops like the Giants or Golems due to their fairly high (over 100 when maxed) DPS or Damage Per Second. 

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       Cannons can ONLY target ground troops. This means that even the lightest air troops like the Minions or Lava Pups can hover over the Cannon and destroy it. This being said you Cannon placement on your village or war base is vital. My suggestion is to group your cannons up with other defenses. An excellent choice will be your Air Defenses. Air Defenses can only target air troops so they need protection from the ground troops. This partnership between these two defenses will allow you to cover both defenses from both air and ground attacks. Be sure to place the Cannon closest to the edge of the base and the Air Defenses behind the Cannon, both within the others defensive circle. (click on the defensive structure to see their range)

       When upgrading your base my suggestion would be to upgrade your Cannons dead last. They may be the cheapest defense to upgrade but they will add the same amount of "Base Weight" to your village as other more versatile defensive buildings. Remember the key to a well rounded base when it comes to your base weight is to add structures that give you the most DPS or that do splash damage 1st. For example your Air Defenses DPS will increase 25-30 every upgrade where your Cannon will increase 9-10 DPS AND they will both increase your base weight at the same rate per upgrade. So for those of us that are constantly trying to increase their bases overall strength without adding tons of base weight we are always trying to upgrade the most effective defenses 1st, leaving the single target defenses for last. An EZE way to upgrade in this manner without a whole lot of calculating is to simple upgrade the most expensive defenses 1st and leave the cheapest for last.