What is Idle Population?
Idle Population refers to the number of residents that are not employed. Although not working, this element of the population contributes tax to the City. Idle Population are also used for training to build your army.
Negative Idle Population means that you do not have enough population to man all of your resources. Your resource output will be decreased until you increase your population to meet the shortfall. You also will be unable to train troops while your Idle Population is in the negatives.
You can increase your total population by providing additional housing or by lowering your tax rate to attract more people to your city. You can adjust your tax rate by clicking on your Castle and clicking the 'Change Tax Rate' button.