The Air Defense has an excellent range and a very high damage per shot. As the in-game description states, it's best to position them so that they cover as much area as possible. One common strategy for Town Hall 7 and above is to position them in a wide triangle (or diamond, once the fourth Air Defense is available around Town Hall 9).
The Air Defense is a popular first target for armies sporting ground troops, as they are one of only 2 defensive structures that cannot target them (the other being the Air Sweeper, which will lose it's synergy with the Air Defense in this case). Make sure they are defended well, and they will help to keep the skies clear from rampaging Balloons, Dragons, Healers, Lava Hounds, and Minions. Air Defenses are there for when Archer Towers and Wizard Towers just won't cut it or are distracted by ground units such as Giants and Heroes.
If possible, keep your Air Defenses behind your first lines of defense (e.g. Cannons and Archer Towers), preferably with a Wall in between. This will prevent a large Giant or Golem assault from quickly destroying the Air Defense and allowing a Healer to be deployed early in the battle. The longer it takes the Giants to destroy the Air Defense, the more time your turrets will have to thin out their ranks before a Healer can be safely deployed.
A good strategy placement for this defense is by placing it near a Wizard Tower (which would be near a Mortar and a storage, for maximum protection).
Spread out your Air Defenses, or else they could all be destroyed with Lightning Spells.
The Air Defense is a natural target to attack first, as clearing them helps pave the way for an air assault. Healers are very vulnerable to the Air Defense, destroying this defense with Giants before deploying her is recommended. They are easily destroyed by any ground-based troop, as they do not attack ground troops.
It is advisable to keep Balloons away from Air Defenses, as they are capable of destroying most Balloons in one or two hits from a great distance.
Although Air Defenses are quite effective against Dragons, it is possible to keep an Air Defense occupied with a one or two Dragons for a fleet of Balloons to quickly destroy a majority of the ground defenses. Since Balloons have a target priority of Defenses, send them close enough to the Air Defenses increases the odds of the AD being easily destroyed.
An Archer Queen backed by Healers is capable of attacking an Air Defense while keeping the Healers out of range. This allows the Air Defense to be taken out without any significant threat to your Healers.
A great way to overwhelm the Air Defense is using a large group of Minions since the Air Defense can only target one air unit at a time and while it does one-shot minions, without AOE defenses/traps to support it the Air Defense is easily swarmed and destroyed.
Air Defenses undergo significant visual changes at levels 2 through to 9.
When initially constructed, the Air Defense consists of a single wooden launch tube with a single red-and-black rocket. The launch tube is banded with metal.
At levels 2, a second launch tube with a rocket is added, with more wood space below.
At level 3, a third launch tube with a rocket inside is added again.
At level 4, the three wooden launch tubes merge into one and gain an additional rocket. The launch base is reinforced with metal.
At level 5, another launch tube is added with 3 rockets in one and 2 in the other. The nail on the right turns golden. These launch tubes seem to be smaller than the level 4 one.
At level 6, another launch tube is added, but there are now 2 rockets in each one. The wood at the bottom turns golden colored and the nail is now spiked.
At level 7, the three launch tubes are replaced with a single large wooden launcher with four large black and yellow rockets in it. The bottom also becomes more secure.
At level 8, the Air Defense's launcher receives a coat of red paint, along with dark metal bands and golden rivets. It is filled with four large, black-tipped rockets that are painted in a yellow and black chessboard pattern, similar to that seen on the V-2 Rocket.
At level 9, the bolts holding the launcher together change to golden spikes, the wooden beams of the launcher become white at the sides and red at the front and possibly the back, and the nails on the launch tube are hidden by golden bands. The yellow bands on the rocket tips become much wider and the rockets themselves become a bit thicker.
At level 10, the launch tube is now dark blue with black straps around it. The base changes to steel gray without spikes, with dark gray beams on the sides; the front beam is now replaced by a bellow-like support with bronze parts on either end. The rockets still use the same color scheme as before, however they have different patterns on both tips and body; no longer using a checkerboard pattern but rather a striped pattern on its body.
The rockets in the launcher look like fireworks. When they explode, you can see a small show of lights if you zoom in close enough.
The visual representation of rockets in the Air Defense's launchers does not represent the amount of ammunition the Air Defense contains. Like most defenses it has unlimited ammunition and will keep firing at enemies if possible.
After the 12/10/16 update, Air Defenses under upgrade have their rocket heads removed and look unarmed. This is made to make it visually realistic, as they can't shoot when being upgraded.
In most English-speaking countries outside the United States, the Air Defense is called the Air Defence.
In Chinese it's known as "防空火箭", or "Anti-Air Rockets".
The number of rockets in level 1-6 Air Defenses corresponds to its level.
Tapping this icon displays information about the Air Defense, such as Level, Damage Per Second, Hitpoints, Range, Damage Type and Targets.
Tapping this icon begins upgrading the Air Defense to the next level, if you have enough resources and a free Builder. When the Air Defense is at maximum level, this icon is not shown.
Tapping this icon instantly upgrades the Air Defense to the next level, at the cost of one Hammer of Building if you have one. When the Air Defense is at maximum level, this icon is not shown.
Tapping this icon, which is displayed only while an upgrade is in progress, instantly finishes that upgrade at the cost of the displayed number of Gems.
Tapping this icon, which is displayed only while an upgrade is in progress, instantly finishes that upgrade at the cost of one Book of Building. This icon is only shown if you have at least one Book of Building.
Tapping this icon, which is displayed only while an upgrade is in progress, boosts all your Builders for 1 hour by a factor of ten, at the cost of one Builder Potion. This icon is only shown if you have at least one Builder Potion.
Tapping this icon, which is displayed only while an upgrade is in progress, cancels the current upgrade and refunds half of the upgrade cost.