I am here today to tell all of you the importance of your Air Defense. First let's go through when you get them. AA = Anti Air (which is Air Defense). Remember Air Defense can only shoot at one target at a time.
Town Hall 4: 1 AA - LV 2 Max - People unlock Balloons
Town Hall 5: 1 AA - LV 3 Max
Town Hall 6: 1 AA - LV 4 Max - People unlock Healers
Town Hall 7: 2 AA - LV 5 Max - People unlock Dragons and Minions
Town Hall 8: 3 AA - LV 6 Max
Town Hall 9: 4 AA - LV 7 Max
Town Hall 10: 4 AA - LV 8 Max
Town Hall 4:
So starting at Town Hall 4 you can buy 1 for 22,500 gold. At this Town Hall you will rarely ever go against flying troops. I recommend getting one, but you do not need to protect it that much. You don't need to upgrade it either yet.
Recommending: 1 AA - LV 1 - Not centralized/Just have it
Town Hall 5:
Your Air Defense become a bit more critical here. Upgrade to at least level 2, and make it sort of centralized. You will start facing lots of Balloons if you don't have it at least inside your base.
Recommending: 1 AA - LV 2 - At least inside your base
Town Hall 6:
Oh boy. Giant-Healer combo is extremely popular this Town Hall. If you don't have a central AA, then you will get wrecked by anyone with a Healer and some Giants. The good thing is, a LV 3 AA will take out a LV 1 Healer in 4 seconds! I highly recommend getting a LV 3 AA in this Town Hall.
Recommending: 1 AA - LV 3 - Centralized
Town Hall 7:
This is starting to get to the point where it seems everyone has Dragons, Minions, Healers, and Balloons to through at you. You also though, thankfully, get a 2nd AA! YEEEAAAaa! Town Hall can be kind of crazy with the pure amount of stuff to upgrade. However, put getting BOTH AA to level 4 a high priority. Also, LV 4 looks very scary to face. If not, then you might as well fly to a different game. See what I did there? Anybody? Ok. :(
Recommending: 2 AA - LV 4 - Pretty central and covering most things inside Walls.
Town Hall 8:
Now it's getting serious. Higher level flying guys are easily available for most. But, you get 3 AA! I recommend the triangle formation just like the Mortars. Get them all the level 5. Even LV 6 if you really feel like it. A common attack is the mass Minion-Balloon attack. Remember, a LV 5 AA will take down a LV 1 Dragon in 10 seconds!
Recommending: 3 AA - LV 5- Triangle formation
Town Hall 9:
Air Defense has never been so important. You get a 4th AA! But sadly this is the point when people have absolutely beast strategies. Don't keep them in the very middle, or they can't touch anything. Do a thing where you put them about halfway out on all 4 sides. DO NOT put them out in the open though.
Recommending: 4 AA - LV 6 or 7 if you feel like it - Quad formation/ Halfway out.
Town Hall 10:
This is the same story as 9. Just here you NEED to have all 4 at LV 7 at least.
Recommending: 4 AA - LV 7 - Same as 9