Go! Go! Tank

Go! Go! Tank
Developer: Copya System Publishers: Copya System (JP), Electro Brain (US) Platform: Game Boy Released in JP: November 30, 1990 Released in US: May 1991 This game has a hidden level select.

Level Select

At the title screen, press Left, Up, Right, Up, Left, Left, Up, Right, Up, Left, Up, then press B a number of times corresponding to the desired level minus 1 (e.g. 3 times for level 4; up to 10 presses). Start the game to begin on the chosen level.

(Source: urawaza.in)