 Random dungeons were created for each floor of the dungeon by selecting ten to twenty rooms from a pre @-@ built library of 200 layouts , adding in the monsters , items , and other features , and then including fixed rooms that would be found on each floor , such as a boss room and treasure room . In expanding the gameplay , McMillen used the structure of Zelda 's dungeons to design how the player would progress through the game . In a typical Zelda dungeon , according to McMillen , the player acquires a new item that helps them to progress farther in the game ; he took the same inspiration to assure that each level in Isaac included at least one item and one bonus item on defeating the boss that would boost the character 's attributes . McMillen also wanted encourage players to experiment to learn how things work within Isaac , mirroring how Miyamoto had done with the original Zelda game . He designed the level progression to become more difficult with the player 's progression in the game , as well as added additional content that became available after beating the game as to make it feel like the game was long . McMillen designed four of the selectable characters based on the main classes of Dungeons & Dragons — fighter , thief , cleric and wizard . 
