As your team fights minions, mercenaries, and heroes, the team gains experience and levels up. Level 20 is the last level you gain any new talent abilities, but your team can keep leveling up beyond that.
What is the maximum level your team can achieve during a match?
All I could find was an old article claiming the max was 25, but that's clearly not correct anymore. I just finished a 40 minute game and my team hit level 28, with the experience bar still increasing: