There truly isn't much left to be said about how bad ass the Double Dragon games were back in the day. They were challenging, yet easy if you develop the necessary proficiency in fighting. They had just enough "special trick/ glitches" in them to keep you talking about the game well after you defeat your first Abobo. 

Double Dragon II is one of those games that, like Contra, encouraged early multiplayer gaming. 

Also, if anyone ever asked me, Double Dragon is ripe for reinvention in the modern era. There hasn't been a game since the SNES epoch.

(And, NO, Double Dragon NEON doesn't count.)