Double Mode is a game mode in BTD6. When this mode is activated, the rounds will be combined. For example, Round 1 will be a combination of Round 1 and 2 in normal mode, Round 2 will be a combination of Round 3 and 4 in normal mode, etc. So there will be only 375 Rounds instead of the normal 750 Rounds. The reward will be multiplied by 1.5 (and rounded to the nearest integer, or ParseInt() if you know what it is) if the game is completed. The game mode doesn't work for apopalypse, because the rounds are continuous and the Round numbers really doesn't matter.