In the combat tab you can buy protection for your city. You can increase the damage, armor and speed of your mercenaries. Also you can increase your available barrack space and increase you tower damage significantly. Some extra information, you are able to buy both Extra Barracks Space-RP and Extra Barracks Space together to further increase your maximum barracks space up to 2916 space. When you buy Protection the is a certain amount of time you have to wait before you can buy protection again.