We started by taking the ruleset that's in the Player's Handbook. We ported it as faithfully as we could, then there were some number of things that we saw that doesn't work that well, and so we started looking for solutions to do that. The hardest part—and this is the most interesting part also about it, because there's a lot of stuff from the rules that actually ports quite well, so—but the most interesting part is the role of the Dungeon Master...
Whatever is not in the book he'll say "Well, I'll do this," and the Dungeon Master says "Sure!" And then he'll think about what type of check he's going to make you do, and then that's going to be what you're going to roll with, and the entire party will work with that. In a video game, you don't have that, so in a video game you have to make systems that allow you to do this. And so, coming up with those systems has been a lot of fun, and making them link to the ruleset as it is has been the interesting bit about that.