How do you beat bloons tower defense

You need to get to a certain level.
This depends on which BTD are you playing on.

BTD1: There is no difficulty. The last level is Round 50.

BTD2: There are 3 difficulties, but they all end at Round 50.

BTD3: Unknown.

BTD4:
Easy: Round 50
Medium: Round 60
Hard: Round 75
Sandbox: No Rounds
Apocalypse: Unlimited

BTD5:
Easy: Round 50
Medium: Round 65
Hard: Round 85
Sandbox: No Rounds
Deflation: Unlimited
Apocalypse: Unlimited