Ninja Kiwi is relatively forgiving compared to Riot or Blizzard, but they are not blind. They employ server-side sanity checks:
Typical Punishments:
You copy the modified file back into the game directory, launch BTD6, and—boom—you’re a millionaire. btd6 save file editor
This is the safest method. Community developers have created tools that decode the save file into a readable interface. Ninja Kiwi is relatively forgiving compared to Riot