Arneson worked to develop and publish the rules for Dungeons & Dragons through TSR, Gygax continued to design and present the dungeons and environs of Castle Greyhawk to his circle of friends and family, using them as playtesters for new rules and concepts. As the players began to explore more of the world outside of the castle and city, Gygax developed other regions and cities for them. With play sessions occurring seven or more times a week, Gygax didn't have the time or inclination to create the map for a whole new world; he simply drew his world over a map of North America, adding new cities and regions as his world slowly grew through ongoing adventures. The city and castle of Greyhawk were placed near the real-world position of Chicago, his birthplace; various other places were clustered around it. For instance, the rival city of Dyvers he placed in the area of real-world Milwaukee.Gygax: "When I was using the pre-World of Greyhawk map for my world setting, the West Coast of North America was the Pleistocene region inhabited by savage cavemen and their contemporary fauna." Gygax also continued to develop the dungeons underneath the castle. By the time he was finished, the complex labyrinth encompassed thirteen levels filled with devious traps, secret passageways, hungry monsters, and glittering treasure. Although details of these original Greyhawk dungeons have never been published in detail, Gygax gave some glimpses of them in an article he wrote for the European fanzine Europa in 1975: Before the rules for D&D were published, "Old Greyhawk Castle" was 13 levels deep. The first level was a simple maze of rooms and corridors, for none of the "participants" had ever played such a game before. The second level had two unusual items, a Nixie pool and a fountain of snakes. The third featured a torture chamber and small cells and prison rooms. The fourth was a level of crypts and undead. The fifth was centered around a strange font of black fire and gargoyles. The sixth was a repeating maze with dozens of wild hogs... in inconvenient spots, naturally backed up by appropriate numbers of Wereboars. The seventh was centered around a circular labyrinth and a street of masses of ogres. The eighth through tenth levels were caves and caverns featuring Trolls, giant insects and a transporter nexus with an evil Wizard (with a number of tough associates)