Clash of Clans: Army Camps

The Army Camp takes up a 5x5 space, making it the largest structure in the game. They continue to function even when they are being upgraded, allowing you to keep your armies at full capacity at all times. Unlike Clan Castle troops, troops stationed in the Army Camp do not defend your village during siege. In fact, they do not appear on the attacker's map at all (all Army Camps, full or not, appear empty to an attacker).

Prior to the Halloween 2012 update, troops would die/disappear if their Army Camp was destroyed in a raid. In the update, troops were allowed to remain even if their camp was destroyed. Due to widespread player support towards keeping this feature, Supercell made it permanent. As a result, troops stationed in Army Camps are no longer at risk if their camp is destroyed by an attacker.

The Army Camp is where your troops go when they're done training. Most troops will take up more than one space (Barbarians, Archers, and Goblins do only take up one space though). Upgrading and building more Army Camps allow more troops to be stored, allowing higher chances of victory in raids.
The maximum amount of housing space you can have for your Troops is 240 (4 Army Camps x 60 spaces). This increases to 275 if you include the extra 35 in a fully upgraded Clan Castle.
The Barbarian King does not take up space in the Army Camps; neither does the Archer Queen. Both Heroes either patrol the village or sleep on their own Altar.

You should not surround your Army Camps with Walls, as they do not need protecting. Also, the Army Camps occupy a lot of space (5 x 5). More walls are needed to contain them when the extra walls can be used to surround defensive buildings that are already inside the wall, strengthening the defense.
It is always preferable to initially build all available Army Camps, and then upgrade the lowest-level ones first. Every time you upgrade (except upgrading to level 2) the camp only gains 5 more housing space and it costs more elixir the more you upgrade. The next level is at least 2.5 times more expensive than the one before it.

Army Camps have a unique hitbox, unlike other Buildings. That means that attacking Troops will have to walk right to the center where the campfire is located in order to attack the Army Camp. The Army Camps can still hold troops while they are upgrading. Clan Castle troops do not go into the Army Camp when donated and do not take housing space in the Army Camp.If you look closely, you can see reactions from some Troops when you tap on the Army Camp. After the Christmas 2014 update the Army Camp's hitpoints has been significantly reduced on all levels.