When users quit the game, I show fullScreen AD.
But I think it's not enough.
Is it correct to add a banner AD at bottom of screen (between those little circle buttons)?
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You need to mention your intentions that what you expect from the users.
If you badly want users to tap on your ad (intentionally or unintentionally) you can place it between the buttons. The present generation using the smart phones are aware of the banner ads and they'll probably be careful. But the older users might just press it mistakenly and that's where you hit the target! For younger users it could be annoying and distracting and they might just uninstall the game, or won't come back if the ad is too close to the buttons they have to use constantly.
But on the other hand, if you want to display an ad for the motive of promotion and not expecting users to tap it unintentionally then that won't be a nice place. You can shift the buttons up and keep a separate place for the banner ad below it.
You can place the ad between the buttons as bottom area of the screen will be in more focus because of these two buttons there. Also, I don't see an issue of being clicked unwillingly as user knows which area of the screen represents the button. Logically, this is like clicking a button on the Physical device say a calculator.