According to Rob Kuntz, this game included both dungeon adventure and outdoors exploration. From what Kuntz describes, tt seems like the experience must have made a considerable impression on Gary:
"After the initial Blackmoor adventure events proceeded at a furious pace. Phone calls to Dave. Letters exchanged between the two. During this time EGG noted that he had begun crafting a “dungeon” setting similar to Dave’s Blackmoor. About two weeks after this adventure, Gary handed me a slim manuscript which had been mailed to him by David. I sat down and read for the first time the rules that David had used during it."
This was of course the beginning of D&D, but it was not the last time for Gygax and Kuntz to play with Arneson as the Dungeon Master. In 1976, those three sat down in TSR's Dungeon Hobby Shop. The two players used their legendary characters Mordenkainen and Robilar. By now, both characters were incredibly powerful and Dave had to limit the number of magical items they were allowed to bring to the table. Their destination was the City of the Gods. Dave Arneson's core group had already made several expeditions to this fabled city, most of which had been disasterous for the player characters. Kuntz and Gygax must have felt confident in their high level characters and played quite recklessly, as Dave Arneson later recalled:
"In this expedition there was a rather indiscriminate and widespread use of lightning bolts which could be observed from a multitude of points throughout the city. The use of such pyrotechnics from a very early stage in the adventure was risky to say the least and was one of the reasons that more and more wandering monsters were encountered thereafter. As the adventurers spent more and more time within the city confines they made little or no attempt to conceal themselves or their activities and so more roving creatures were drawn to their ramblings."
-Dave Arneson, Oerth Journal #6
This lack of caution nearly cost Robilar his life. Fortunately Mordenkainen was able to save his friend and the two were among the few to have survived exploring this deadly place. Details of this adventure can be found in Oerth Journal #6.