Fortified Zone for the Game Boy has been released for the Nintendo 3DS Virtual Console in North America today. It'll cost you $3.99:
"Originally released in 1991, this action game allows two characters to be used alternately. Secret agents Masato and Mizuki have been dispatched to infiltrate and destroy an enemy fortress."
Also available are Qix (Game Boy, $2.99), Make Up & Style (DSiWare, eShop, $4.99), Roller Angels (DSiWare, eShop, $4.99), and Farm Frenzy (DSiWare, eShop, $4.99).