One of the perennial questions about the history of role-playing games is this: which came second, Tunnels & Trolls or Empire of the Petal Throne? Deciding between the two is largely a question of semantics, of whether you count various small-run amateur publications as releases or not. Fortunately, historians don't need to decide between the two, because Craig van Grasstek's Rules to the Game of Dungeon (1974) beat them both handily. Weighing in at eighteen pages, and released late in the summer of 1974, Rules to the Game of Dungeon seems certain to be the second published role-playing game.
The great irony of Grasstek's Dungeon is that he was unaware of Dungeons & Dragons when he produced it, due to an almost incredible chain of events. In February of 1974, a Minneapolis science-fiction fan named Louis Fallert (better known as Blue Petal) began running a game called "Castle Keep" which he based on his experiences playing in dungeon adventures with the MMSA (Fallert wrote a blurb about this for Alarums & Excursions #3). While it seems likely that Fallert himself was vaguely aware of Dungeons & Dragons, he presented "Castle Keep" to local science-fiction fans as a game of his own invention. Much as Gygax ran Blackmoor with Arneson and then largely implemented his own rules from his experience, so did Fallert build a system for dungeoneering that followed his own subjective impressions as a player.
These "Castle Keep" games became quite popular in Minneapolis in 1974, as contemporary issues of the local fanzine Minneapa document, and many local fans developed their own dungeons, largely unaware of the existence of Dungeons & Dragons. Craig van Grasstek played in the first incarnation of "Castle Keep," and later ran a dungeon; as the Foreword to his rules observes, there were nine total dungeons based on "Castle Keep" in Minneapolis by the summer. Grasstek recognized that there was little standardization among the rules of these dungeons, but he nonetheless hoped to write up an account of the game that would let people outside Minneapolis share in the fun. He therefore brought fifty copies with him to the World Science Fiction Convention at the end of August 1974, and circulated his rules through Minneapa.
Once he started publicizing his rules, others were quick to point out the existence of Dungeons & Dragons, and thus Rules of the Game of Dungeon quickly fell into obscurity. It did however influence a generation of dungeoneers who began playing in 1974. Glenn Blacow, later a very prominent early player and dungeon master in Boston, first learned the game from Grasstek's Dungeon before he ever saw a true copy of Dungeons & Dragons.
Fallert clearly was not trying to rip off TSR, and neither was Grasstek - neither profited from this venture. Both were simply sharing a pastime they enjoyed, in a manner that is well in keeping with the practices of science-fiction fandom. Grasstek only inadverantly released a game that competed with Dungeons & Dragons; it would not be until 1975 that other authors would intentionally try to market simpler and cheaper alternatives. In fact, it is likely that by preserving the game as he did, Grasstek did historians a great favor. There are certain elements of his Dungeon that resemble accounts of the original Blackmoor system, and it seems likely that Fallert informally preserved some Twin Cities practices for play that did not end up in the final drafts of Dungeons & Dragons. Thanks to Grasstek, we have a unique window into strongly reagant-based magic systems, mitigiation-based armor and much less formal dicing mechanics.