Once the seat of the Commonwealth's government, Beacon Hill was called "Trimountain" and later "Tremont" by early colonists because of its three summits, Pemberton, Mt. Vernon Hill, and Beacon Hill, and named for the warning light set on its peak in 1634. In 1799 settlers leveled out the ground for residences, using it to create what is now Charles Street; by the early 19th century the crests of the other two hills were also lowered.
When the fashionable families decamped for the new development of the Back Bay starting in the 1850s, enough residents remained to ensure that the south slope of the Hill never lost its Brahmin character.
By the mid-20th century, most of the multistory single-family dwellings on Beacon Hill were converted to condominiums and apartments, which are today among the most expensive in the city.
A good place to begin an exploration of Beacon Hill is at the Boston Common Visitor Information Center , where you can buy a map or a complete guide to the Freedom Trail.