Cannons are a single point defense unit. The are great for taking out larger ground troops such giants and pekkas. Cannons cannot attack air units.
Archer towers are also a single point defense unit. However, unlike the cannon, archer towers can attack both air and ground units and also have longer range than the cannons too.
Walls are built to keep enemy ground troops at bay from your buildings and resources. The stronger the wall, the longer it takes to break down. Walls only stop ground units. Air units will fly over walls unharmed and unstopped.
The mortar is a firm defense favourite. It does splash damage meaning it will take out enemy units within a certain area. Great for defense against barbarians, archers and goblins. They can only take out ground units and not air units. Mortars are unable to attack units that are too close to themselves.
The bomb is a single use defense unit. When enemies step on the bomb (which is hidden from the attacker), it blows up and causes damage to a small area.
The air defence tower is designed to take out flying air units only. It will not attack ground units. It is very effective is tackling all flying units, even the mighty dragon.
The spring trap tosses (ejects) enemy units that stand on them. These are invisible to the attacker and are unable to harm air units and pekkas ( as they are too heavy ).
One of the most powerful defense units. The wizard tower inflicts splash damage and can defend against both air and ground enemy units.
The air bomb, as the name suggests, is a bomb against enemy air units. Once an air unit flies above one of these ( again hidden from enemy view ), it opens up a hatch and the seeking air bomb collides with the enemy and blows up.
A more powerful and more expensive version of the bomb. The giant bomb can easily take out a hoard of archers, barbarians and goblins if triggered.
The hidden tesla is hiddn until and enemy unit comes within range. When it does, it springs up from the ground and zaps electricity at both air and ground units as they pass by.
Like the air bomb but more powerful and more expensive. The Seeking Air Mine requires dark elixir to buy, but are very effective against powerful air units. But as said, it comes at a price.
Seeking Air Mine
The x-bow is an elixir filled machine gun. It can be set to target just ground troops (extended range ) or both ground and air units ( smaller range ). The x-bow needs refilling with elixir and will stop working if it has run out.
The clan castle houses troops donated to you by your fellow clan mates. These troops help to defend your base when an enemy attacks.