You can unlock old stages from the original Lethal League by playing a certain number of online matches.

Unlockable	How to unlock
Abandoned Pool (Retro)	Play 5000 matches.
City Streets	Play 500 matches.
Hammer Express	Play 4000 matches.
Industrial Outskirts	Play 6000 matches.
Room 21 (Retro)	Play 3000 matches.
State Manufacturing Factory	Play 1000 matches.
Underground Sewer System	Play 2000 matches.