The Main Army is the army where your country tries to gather all its players together. It is also where you usually will be safer while being offline. You can only join it when you're blue ranked ( TLK: Lieutenant and higher, KnC: 3rd Lieutenand and higher).
The main army is also used to defeat other main armies. When this occurs, your main army will attack a border city containing an enemy main army, and battle each other. When the defending main army loses it gets kicked to their capital, and the main army is disbanded and has to be rebuilt ( when online, the people need to rejoin it). Main drops usually are used when the enemy is down to 1 city, and all its players are in that city.
How to join the Main army:
1) Click city centre.
2) Click visit castle and look in what city your main army is located.
3) Travel to that city.
4) Click city centre.
4) Click visit castle and there should be a join button next to the main army.
5) Join it.