From Deepolis Wiki

Gameplay

Level

A player's level shows their current progress in the game and their strength as well. It is displayed in square brackets right behind a player's name. Example: Opponent[50]

In order to advance to the next level, a player needs to collect Experience Points (EP). Leveling up has many advantages; every player causes 1% more damage per level with their Sonic Cannons, and by reaching higher levels, a player can purchase new and improved items.