Back in the days when dinosaurs ruled the earth and Nintendo ruled the games industry, third parties had to jump through a lot of hoops to get their software onto the Nintendo Entertainment System. Nintendo was not a kind and benevolent god by any stretch of the imagination, and publishing games for their money factory required compromise.
That's how we ended up with games like Double Dragon. The NES version of Technos' seminal arcade brawler barely resembled the coin-op original. Some details, like the radically revamped graphical style and change to single-player-only action, were simply unavoidable side effects of the diminished hardware power offered by the NES. Others, however, came about as a result of the fact that Nintendo demanded exclusive content for its console.
Publishers had two choices if they wanted to bring a huge arcade hit home and dredge money out of the vast NES money pit: They could put the arcade conversion exclusively on NES and forego all the other platforms available at the time, or they could port the arcade version to every platform but NES and offer Nintendo fans an exclusive reworked version that barely resembled the coin-op. Generally, developers took the latter course; the NES was big, but passing up on all those other platforms' potential audiences would have been a dumb business move.
Sometimes the results were great; the Ninja Gaiden NES game was far better, more memorable, and more influential than the callow arcade version. Other games didn't fare so well; Capcom gave Strider the ol' college try, but the flaky adventure thrust upon Nintendo fans lacked the appeal and oomph of the coin-op (whose direct conversion became a key early selling point and "look how much better this is than your stupid old NES" talking point for Sega's Genesis). Double Dragon, then, fell somewhere in between. Better in some ways than the arcade rendition and worse in others, it's probably best regarded as an oddball experiment that in some ways was ahead of its time.
For starters, Double Dragon came with a crazy bonus variant called Mode B. This wasn't just a throwback to the early days of "black box" NES software where the A and B versions of games generally amounted to difficulty levels, though; Mode B was in fact one of the first-ever arcade-style brawlers. Players could select jumbo-sized versions of Double Dragon's characters (including a bunch of the enemies) and go head-to-head with either the CPU or another player, pulling off authentic moves from the game in a small urban combat arena. Though presumably designed to compensate for the lack of co-op play in the main game, Double Dragon's one-on-one mode was totally unique to the NES and, coming a mere year after the original Street Fighter, seems rather forward-thinking.
Nevertheless, Mode B was fairly shallow and limited in nature, with no special or combo moves. The bulk of the experience came in the main game, a loose one-man interpretation of the famous arcade walk-and-punch brawler that set the medium on fire. Double Dragon-style games ruled the arcade for several years until Street Fighter II came along (meaning this version of the game shipped with the seeds of its own obsolescence built right in!), and a big part of their appeal came from the ability to team up with a friend and beat the everloving stuffing out of an army of goons. Double Dragon abandoned that hook for the NES, existing entirely as a solo experience.
With that fundamental aspect of the coin-op's appeal lost, Technos had to compensate somehow. The programmers did a decent enough job of reworking the arcade's visual style to NES; everything became brighter and cuter, but it all looks detailed and consistent. At the time of its release in 1988, Double Dragon was possibly the best-looking game on the system, albeit considerably more day-glo than its upright counterpart. But the real change in Double Dragon came from the skill system, a primitive take on RPG experience and leveling mechanics.
As Billy Lee -- player 2's Jimmy Lee had been moved to the end where he now played the role of unambiguous villain -- players began with only a handful of skills. But performing attacks would cause a "heart meter" on the left side of the screen to increment upwards, and every 1,000 "heart points" Billy would level up. With each new level would come a new move, from standard finishers (uppercuts and roundhouse kicks) to bread-and-butter moves like the jump kick. All the techniques of the arcade game (and more) were included in the NES game, but you had to make the effort to unlock them.
It was a pretty interesting idea that would be executed much more effectively a couple years later in Technos' own River City Ransom. In practice, Billy's leveling mechanics meant you'd have it a little rough at the start with only your basic skills available... and then once you unlocked power moves like the jump kick and whirlwind attack, you could blast your way through foes designed to be manageable with lower-level abilities. It also introduced a world of American kids who didn't really know what an RPG was to the concept of grinding; it was entirely possible to exploit the knockdown mechanics of Double Dragon to keep a low-level enemy alive to use as a punching bag until the clock ran down in order to power-level at the game's very first screen.
Besides the level system, Double Dragon offered an interesting take on the arcade game in other ways. In being redesigned for solo play, it tended not to overwhelm the player with massive encounters, but instead integrated more platforming challenges into the action. The final stage, the enemy fortress, proved to be especially maddening in this respect, with moving elevators and an insane trap wall that pushed the game's design and controls well beyond what they were designed to support. With two lives and no continues, the randomized death wall in the final stage could bring a hard-fought adventure to a cheap and untimely end -- a crummy way to go after grinding up all those heart skills.
While uneven to a fault, Double Dragon on NES represents an ambitious effort to take a tough business situation and do something innovative with it. While Technos' efforts didn't quite pay off, Double Dragon seemed pretty amazing back in 1988; and with its experience system and more complex level design, it was as influential in its own way as the coin-op game.