Robotron Codes -

Level select:
Press Down, Up, C-Left, Down, C-Left, C-Right, Down, C-Right at the setup screen.

Fifty lives:
Press Up(2), Down(2), Left, Right, Left, Right, C-Left, C-Right, C-Left, C-Right at the setup screen.

Monochrome mode:
Press Up, Down, Right, C-Left, Down, Up, Left, C-Right, Up, Down at the setup screen.

CPU control:
Quickly press [Left, Right, Up, Down, Left-C, Right-C, Left-C, Right-C] three times at the setup screen.