Endless Zone

The Endless Zone is an game mode about infinite rounds in Bloons Tower Defense: A Doctor's Tale

You unlock the Endless Zone of its respective world everytime you complete a world.

Endless Zone: Grassy Pass
In this Endless Zone only the following bloons appear: After a long amount of rounds, only green bloons will spawn and in huge amounts.
 * Red Bloon
 * to
 * Green Bloons

Endless Zone: Bloon Dunes
In this Endless Zone only the following bloons appear: In the later rounds, only pink bloons spawn.
 * Red Bloons
 * to
 * Pink Bloons

Endless Zone: Extreme
In Endless Zone: Extreme, all kind of bloons appear (except for secret ones if not discovered yet)

and randomly one round may be taken by a boss of Adventure Mode. The more higher the rounds are, the stronger are the bosses. Eventually you get the final boss and then the boss repeats but now two bosses appear in the round. And so on.

This mode is unlocked when Adventure Mode is completed.