Levels are the space where a player explores the rules and mechanics of a game; as such, good level design is critical to the game design process. While there are many broad design principles, level design is inherently genre-specific due to the wide variety of rules and types of challenge found between genres. Determining genre-specific design principles requires an in-depth analysis of games within the genre. We present such an analysis for the 2D platformer genre, examining level components and structure with a view to better understanding their level design. We then use this analysis to present a model for platformer levels, specifically focusing on areas of challenge. Our framework provides a common vocabulary for these items and provides level designers with a method for thinking about elements of platformers and how to compose them to create interesting and challenging levels.