The Rally Spot is a building for your army to live in, where you can give an order to attack or conduct a military exercise.
The higher the level, the more troops you can dispatch at a time.
Level 1 you may send 10,000 units to attack a target.
Level 5 you may send 50,000 units to attack a target.
Level 10 you may send 100,000 units to attack a target.
You get the point.
What does opening my gate do?
The “Open Gate’” feature determines whether your army adds its strength to the city defenses, or stays safely in barracks to leave the wall defenses to take on an attack unaided.
If you open the gate, your army will help defend your city but you also risk losing the troops.
If you close the gate, your troops will stay put as an opponent attacks you, but you have no way of losing your army save losing your city.
What is the Medic Camp?
In your Rally Spot you will find a button that says Medic Camp. This holds all of the troops that have been wounded in your recent battles. You may heal your wounded troops for a certain gold fee, or else they will eventually all die.