These are the gamemodes that are currently in the game
It's just classic tower defense gamemode. You place towers and try not to die from waves of cubes. Usually seen in most TD games. It has three modes to it:
Most likely an idea from BTD 5, it is the same as normal Classic mode, except that there are up to 4 other players helping you. I forgot whether you share cash, or that you each have your own little wallet of cash.
Most likely an idea from BTD Battles, you have to face off against another player. You place towers to survive against rounds that do not have any modes to them. You could also send cubes to attack the other player(s).