How do I repair my training ground?

The training ground itself is a building and can only be upgraded. Each training ground however does control one of your legions and tapping on the training ground will show a 'Repair' option.

Each legion has an amount of durability, this is used up when you send it out on marches, to attack or gather etc. You will have a max of 100 durability and when you use it up you can no longer send out marches. The durability will recover by itself over time, but you can also 'repair' it quickly by using stamina potions.

The only way to get these though is to buy them for real money. So the repair option lets you use the potions to recover stamina immediately instead of waiting.