It could easily be argued that video roleplaying games wouldn't exist without Dungeons & Dragons. Early computer titles such as Dungeon, Akalabeth, Wizardry, and the Ultima series all took many ideas and game mechanics from the pen & paper game. As did early console RPGs such as Dragonstomper, Phantasy Star, Dragon Quest, and Final Fantasy. Over the years, dozens of videogames have been made based directly on the D&D license. Some were great (SSI's Gold Box series, Baldur's Gate, Neverwinter Nights, Planescape: Torment) and others not so great (Iron & Blood: Warriors of Ravenloft, Heroes of the Lance). Due to the time involved in raising your characters' abilities, exploring fantastic realms, collecting rare and magical items, and embarking on epic quests to save the world, RPGs were best suited for playing at home over extended periods. The last place you would expect to see a roleplaying game is in the arcades. Capcom, never one to listen to reason, actually wound up releasing two coin-op titles: 1993's Dungeons & Dragons: Tower of Doom and 1996's Dungeons & Dragons: Shadow Over Mystara.
Capcom USA employee Alex Jimenez was the creative force behind the D&D arcade games and his years of experience as a Dungeon Master was put to good use. Jimenez initially created the story for Tower of Doom as a traditional D&D adventure and tweaked the balance of the arcade title based on feedback from his players. Both games were at their core standard side-scrolling hack & slash, easily recognizable to gamers who previously dropped quarters into Capcom's Knights of the Round or The King of Dragons. While these earlier titles included minor RPG elements such as character leveling through experience and item upgrades, Tower of Doom and Shadow Over Mystara truly manage to capture the flavor of a genuine D&D adventure.
Tower of Doom allowed players to select between the character classes of Fighter, Elf, Cleric, and Dwarf, while it's follow-up Shadow of Mystara expanded on these choices with the addition of the Thief and Magic-User. Just as in most RPGs, each class had their own unique strengths and limitations. Characters such as the Fighter and Dwarf were your no-nonsense combat monsters, the Cleric and Elf blended fighting prowess with offensive and defensive magic spells, and the Thief and Magic-User were specialists that made up for their physical weakness with distinct skills and impressive damage-dealing ability. Combat is not much different than your typical arcade beat 'em up, but these games (especially the sequel) feature a greater depth than normally seen in a coin-op. One nice touch is replicating saving throws against attacks such as petrification by waggling the joystick.
Throughout the adventure, players would collect treasure as well as pick up magical items such as rings, armor, and even legendary weapons, which could be used to enhance your character's attributes. By visiting the local shopkeeper between each level, players could stock up on healing potions as well as secondary weapons such as throwing daggers, hammers, arrows, and flaming oil pots. Spells were handled in a similar way to the pen and paper game, you received a limited number of uses of each spell type augmented by any scrolls you happened across. If you burned through your magic too quickly, you'd be forced to go hand to hand until you completed the stage or lost your life.
Staple creatures straight out of the Monster Manual were your fodder in each level, including kobolds, goblins, skeletons, hellhounds, dark elves, gnolls, and owlbears. The boss battles featured deadlier foes such as displacer beasts, trolls, beholders, liches, ogres, chimeras, and of course a variety of chromatic dragons. The names of various spells and magical items would be familiar to any D&D player, such as Cure Light Wounds, Magic Missile, Gauntlets of Ogre Strength, or Ring of Protection +1.
The story for both games is your standard "rag-tag group of heroes must defeat overwhelming evil to save the world", but what made these different was the chance to select different paths during the game. In one example, the players would encounter a wounded solider. He'd give you the option to rush to defend a fort under attack, or instead travel to a nearby city to request more reinforcements. The choice you made would determine the future stages you visited, bosses you encountered (such as the incredibly difficult red dragon Flamewing), and magical items you could ultimately receive. Fully experiencing either title would require several playthroughs with multiple characters.
In 1999, both titles were ported to the Sega Saturn as the Dungeons & Dragons Collection. Unfortunately, due to poor sales of the Saturn in North America and Europe, this compilation was never released outside of Japan. This version is extremely rare and lack of English text provides an additional obstacle to importers. A PC port of Shadow Over Mystara was released by Korean company MCB Interactive in 2003, but is out of print and quite difficult to track down. The D&D license is currently held by Infogrames under the Atari name, so legal entanglements make it highly unlikely we'll see a release of either of these games anytime soon. As awesome as a four-player online XBLA or GameTap emulation of Tower of Doom and Shadow Over Mystara would be, MAME is probably your best bet if you'd like to experience these for yourself.
A great amount of information on both games was found here, as well as various other pages spread across the InterGoogles.
Licensed Games That Don't Entirely Suck - #1: DuckTales
Licensed Games That Don't Entirely Suck - #2: Chip 'n Dale Rescue Rangers
Licensed Games That Don't Entirely Suck - #3: Super Star Wars Trilogy
Licensed Games That Don't Entirely Suck - # 4: The Dungeons & Dragons Arcade Games