Spelunky began as a freeware roguelike platformer for the PC, but has evolved into a larger-scale XBLA title due out this year.

“When I think about how that happened, how I started with this tiny little game, and it became something much larger, I’m still pretty amazed,” said Derek Yu, the game’s creator during an Independent Games Summit talk at GDC 2011.

Before making Spelunky, he had wrapped up a large project. “I really wanted to work on something a lot lighter,” said Yu, “something small and kind of fluffy. It’s kind of ironic because Spelunky’s now turned into kind of a monster.”

To get there, he made a lot of small games in between the bigger ones. “When I think about it, all those small games I made turned into something bigger,” he said. “They gradually got bigger, and that got me the confidence to do something better.”

Yu considers the freeware version of Spelunky to be a prototype for the XBLA version. He likens prototyping to drawing. “The key is really to zone in on what I’m enjoying at the time, and things I’m interested in,” he said, adding that tools like gamemaker and unity “are the equivalent of a sketchbook for me, because they help me get my ideas out as quickly as possible.”

Spelunky came about because he was prototyping a roguelike and a platformer simultaneously, and with both of them he “really didn’t get very far,” he said. “I came up with the idea of combining the aspects of roguelikes that I liked and found compelling. I wanted to combine that with a platform game.”

“I wanted to make something that was new and exciting, but also accessible,” he added, saying that with procedural content, “you can create a lot of content with a small team.” Spelunky’s levels are randomly generated from a set of specific room-types, but with additional randomization within them. “It’s really fun to work this way, because you’re always surprised by your own game.”

Yu is a big believer in the idea of prototyping game ideas. “Chris Hecker, who I know loves to be quoted out of context, described games that don’t reach their full potential as mulch, and games that do, as trees,” Yu said. “I really liked that idea, and thought you could make mulch and trees at the same time.”

Andy Hull, programmer on the XBLA version of Spelunky, said that they’re treating the game as more of a sequel to the original version. “This might be scary to hear, but you can’t be afraid to change things from the original game,” he said. “This was made easier by the fact that the original game will still exist.”

The most important thing in the new version, aside from the new art, is the streamlined interface. “When I first played Spelunky I didn’t stick with it for very long, because I had some difficulty with the controls,” said Hull. It wasn’t until he saw how much people liked it on TIGsource that he went back to it.

Previously going to the shop keeper was difficult, with multiple button presses required to purchase items, and the possibility of accidentally angering the shopkeeper. So they streamlined it, and now buying items requires a single button press.

One feature that was more requested than any other, and that’s multiplayer. When they announced four-player local play for the XBLA version, the audience erupted into applause.

“The freeware game acted as our design document for the XBLA game,” Hull concluded. “Spelunky really shows that there’s more benefit to releasing these [prototype] games than people realize.”