The Middleman’s job is watch your template files (Haml, Sass, Erb, Builder, Maruku & Markaby) for changes and provide a development environment for rapid site creation and prototyping. Some important features are layouts, ruby helpers and the integration of the Compass CSS library. Middleman is built upon Sinatra. In fact, the Middleman init.rb is evaluated within a Sinatra context so anything you could do in Sinatra can be done in Middleman.

The Sinatra Book (http://www.sinatrarb.com/book.html) has some great examples for creating helper functions and changing the router.