Thank you for the feedback :) We're really between two minds about an undo button. Almost everyone seems to want one, but as the commenter just below you said, if we add an undo button then our confusing controls aren't a feature anymore they're just bad controls.
We'll defo have a hard think about controls and undoing if we make it into a full game (which we might; its the only actually-small idea for a game we've had and people seem to enjoy it and we have ideas for how to expand it)