Have you ever attacked someone, used your spells and clan troops, and then gone offline? Everyone has. Well, imagine you did that, and when you were offline, someone swooped in, grabbed your loot, and 3 starred you, because you hadn't gotten more troops from your clan to help defend your base. This has happened to me many times.
This building, the army garrison would be able to store your own units, or units moved from your clan castle. The units would not take up space in your army camps, but they would also not be able to participate in attacks. All they would be used for would be defending your base when you are attacked. Units that are not destroyed would be able to return to your garrison, and defend in the next attack.
If it was just left at that, it would be a little overpowered. You would have troops shooting out of both locations, quickly killing the defenders. This is why the troops that are in the clan castle would be no longer used for defending, unless they are moved into the garrison by the player. I also know some people who do not see the need to join a clan; this would allow them (and others who don't feel it is necessary to join a clan) to have troops to defend their base, though they would need to build the troops themselves.
This structure would probably be available at TH 5. It could have similar unit storage to the clan castle, 10 at level 1, 15 at level 2, 20 at level 3, 25 at 4, 30 at 5, and 35 at 6. You would be able to level up the garrison once for every time you have upgraded your TH. This would mean that you could not upgrade to level 2 until you have upgraded to TH 6, level 3 unlocks at TH 7, and so on, for 6 levels.
