The Binding of Isaac started in a weeklong game jam. Tommy Refenes (Super Meat Boy co-developer) was taking a vacation, so I decided to do the game jam with Florian Himsl, who programmed a few of my previous Flash games (Triachnid, Coil, and Cunt). Florian is the kind of guy who is up for anything; he wasn't worried about his reputation, and was basically down with whatever I wanted to do in terms of content. This was good, because I had two clear goals when I started designing Isaac: I wanted to make a roguelike game using the Legend of Zelda dungeon structure, and I wanted to make a game about my relationship with religion. [...]

1. Roguelike Design

The roguelike formula is an amazing design plan that isn't used much, mostly because its traditional designs rely on alienatingly complicated user interfaces. Once you crack the roguelike formula, however, it becomes an increasingly beautiful, deep, and everlasting design that allows you to generate a seemingly dynamic experience for players, so that each time they play your game they're getting a totally new adventure.

I wanted to combine the roguelike formula with some kind of real-time experience, like Spelunky, but I also wanted to experiment more with the traditional role-playing game aspect of roguelike games Crawl and Diablo. Fortunately, using the basic Legend of Zelda dungeon structure as the game's skeleton made it easy to rework almost all the elements of a traditional roguelike formula (procedurally generated dungeons, permadeath, and so on) into a real-time dungeon crawler format. Almost every aspect of the game seemed to fall perfectly into place with little effort.

Let's start by looking at the Legend of Zelda dungeon and resource structure -- it's simple, and really solid. Keys, bombs, coins, and hearts are dropped in various rooms in the dungeon, and the player needs to collect and use these resources to progress through each level. In Isaac, these elements were randomly distributed and not required to progress, but I included them to add structure to the experience.

I also pulled a lot from Zelda's "leveling structure," where each dungeon would yield an item as well as a container heart to level up the character and give the player a sense of growth; in Isaac, each level contains at least one item, and the player can get one stat-raising item by beating the boss. These items are random, but still designed in a way that made it so your character would have some kind of physical growth as you progress through the game.

I approached the roguelike design from many different directions with Isaac, but at its core, what made Isaac different than most roguelike games (well, aside from its visuals) was how I dealt with the difficulty curve. Instead of using traditional difficulty settings, I simply made the game adjust to players as they played, adding increasingly difficult content to the game as they progressed. This made Isaac feel longer, richer, and gave it the appearance of a story that writes itself. Using this design also allowed me to reward the player for playing and playing well, with more items that would help aid in their adventures and keep the gameplay fresh and exciting.

Once the player finally overcomes Mom, they usually assume the game is over, but instead get a new final chapter, six new bosses, a new final boss, and new items that shuffle into the mix. When the player beats the final chapter, they unlock new playable characters and items, and when they beat the chapter with each new character, they'll unlock even more content that makes the game even deeper still.​