Nice question, Warrior!

Let's take a look on this subject:

Along the game, every time you upgrade a card, you earn experience points.
Once you have enough experience points, the player level will automatically upgrade, improving your bunker, small bunker, and trenches attributes.

Sometimes, you may be concerned about the difference between your opponent's level and yours, or the cards your opponent has, but the game only considers the number of medals to do the matchmaking.

Also, the player’s maximum level is 17.

See you on the battlefield! ;)