At PlayMountAiry, you can increase your Player Level by earning Experience Points. Experience Points are gained every time you wager coins in a game. By increasing your Player Level, you unlock additional Slots Games.
You need to reach the corresponding Level in order to unlock that game. For example, you need to reach Level 20 in order to unlock High Flyer's Spooky House, as per the image below.
You can view your Player Level, as well as your Experience Bar, at the top of every game screen.