You can download and play Airport City for free, but there is an in-game currency that is used to speed up the game process. The in-game currency can be purchased with real money via the App Store, Google Play, Amazon, Samsung Galaxy Apps, and Windows Store.

To disable in-app purchases on your device, follow the link next to the store you use:
for Google;
for Apple;
for Amazon;
for Windows.
for Galaxy Store.

If you want to keep purchases enabled, set up the device so that it requires a password whenever you try to make a purchase (select “Always require“). You can also set a window of 15 minutes from your last purchase after which you will have to re-enter the password.