Xonox is a 3rd party company that had an innovative idea. They released cartridges with 2 games, one on each end called a Double-Ender. Games were decent to play, with some cool titles. Chuck Norris had a game created based on his Kung-Fu populatiry. There were also games based on legends Robin Hood and Sir Lancelot. Artillery Duel was cool in the fact that you used mathematics to shoot cannons at various angles. A few of the games are common, and can be had for $8-15. These are double ended cartridges. The rarest cartridges to be had for the most part are the games released as single cartridges. There are only 8 different games released by Xonox, but there are a combination of 17 cartridges in either single or double ended form. From a player standpoint, you should be able to acquire a copy of the game for under $25, though particular single or combination cartridges may run up to $100 or so. There were cartridge color variations on some of the games, either plastic was grey or black. See individual listings for variations.