[TOMT][Book] Sci-Fi story with an alien theme park with multiple layers of virtual reality.

I read this book no later than the late 1990s. If I recall correctly, the setting was an alien planet, with some sort of virtual reality entertainment system. Humans had discovered the planet after the creators were long gone, and started playing with things. Inside the VR system, there were multiple (three I think) layers of game, and you could move from layer to the next. The three were like three different genres of game.

The deepest layer had some sort of demon summoning "magic" system in it. Near the end of the story they discover that the magic isn't really part of the VR system. Instead, the entire system was in place to hide the fact that this magic really worked.