Challenges
Alien Creeps TD Guide
There are 37 Challenge maps in the game, with three difficulties each. Challenges are extra missions that can earn you extra coins as rewards. Challenge maps are basically re-used campaign maps with no blocked tower spots and an unknown number of waves per map.
The two key features that distinguishes Challenge missions from normal missions is that your Tesla Tower doesn't have any charge (though you can spend gems to recharge it; I'm advising against this since gems are very valuable) and the starting energy is about half of what you normally get from normal missions. You'll only have three lives as well.
They are unlocked as you clear maps and progress deeper in the game. You cannot "farm" coin rewards from missions since they are only awarded once upon clearing the challenge. You can still replay the mission but with no rewards.
Also, there are tied achievements to their progress, which will allow you to earn even more coins.
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