A Pay Wall is a point in the game where you must pay money to continue playing the game. If the game makes this clear ahead of time, then this isn't a Dark Pattern. Perhaps the first 5 levels were a free trial to see if you like the game and then you can purchase the rest. This is essentially the "Shareware" model of game development which is not a dark pattern. A similar practice is Downloadable Content (DLC). With a DLC, you purchase additional content that extends the standard game that you already have. If the DLCs are known ahead of time, this is a clean practice.
It becomes a Dark Pattern when the game doesn't disclose this ahead of time and surprises you with this information after you have been playing for some time. If you have played part of the game already, it may have some endowed value
for you, or you may have a partially completed goal
that you want to finish. If the game suddenly stops working until you pay to unlock the complete game, you may have a strong incentive to pay. To avoid this dark pattern, games should be up-front about the cost and how much you can pay for free.
A game may also contain a secret hidden Pay Wall. This occurs when the entire game is free, but the difficulty increases steadily until it's nearly impossible to continue without purchasing in-game items or upgrades that make it possible overcome a particularly difficult level or obstacle. The game doesn't directly prompt players for payment, but instead relies on their growing frustration with being unable to beat a level to drive sales of in-app purchases