Ishihara: [...] Mystery Dungeon games have their roots in old-school RPG titles, the type they often call 'roguelike' games.
Iwata: Ah yes. 'Roguelikes' are dungeon-exploring RPGs in which the map is altered every time you play, with the terrain and the locations of items and monsters also changing. This gives the games a depth that means you can play them over and over again without getting bored. The game that was really crammed full that kind of enjoyment, while being accessible for everyone to play, was Torneko no Daibōken: Fushigi no Dungeon.
Ishihara: At the time, the tagline for the game claimed that it was a "game you could play a thousand times" and I think that I, for one, really did! After I started working on the Pokémon games, therefore, I was always really keen to create a game that would connect Pokémon to the Mystery Dungeon series. Pokémon Mystery Dungeon was the game that arose from that desire.
Iwata: Nagahata-san, what kind of process did you go through, transforming the idea of a roguelike game into the Mystery Dungeon series? When you first set out, you didn't know how a game like that would be received by home console owners, did you?
Nagahata: No, we didn't. At that time, Dragon Quest-style RPGs were the mainstream and very little was known about roguelike games, generally. So it was definitely a bit of a gamble. After all, players would sometimes play roguelike games for hours or even tens of hours, only to suddenly be dumped right back in square one with all their progress wiped out. Basically, however, all the development staff told us that this kind of game was definitely enjoyable, and that gave us the motivation we needed to get started.
Iwata: There's certainly an element of 'spiritual training' to these games, isn't there? They're constructed in a really enjoyable way, but every now again they're just so unforgiving that they make you want to cry. You were really motivated to bring that particular brand of enjoyment to as many people as possible, though, weren't you?
Source: Iwata Asks on Pokemon Mystery Dungeon