Complex sentence: Beacon Hill has three sections: the south slope, the north slope and "Flat of the Hill", which is the level section of the neighborhood built on landfill that is west of Charles Street and between Beacon Street and Cambridge Street.
Split into simpler sentences: Beacon Hill has three sections: the south slope, the north slope and "Flat of the Hill", which is a level neighborhood built on landfill. It is west of Charles Street and between Beacon Street and Cambridge Street.