The Colonial Inn, a combination of three separate buildings representing important developments in local history and commerce, dominates the northern end of Monument Square. Built before 1716, the eastern section is the earliest, despite the later appearance that its mansard roof addition (1860s) suggests. The center section, originally a single-story storage unit built in 1775 to hold the munitions and supplies being assembled in Concord, was expanded around 1800 with a second level and hipped roof. The western third, built between 1812 and 1820, is a five-bay, two-story hipped-roof complex with a projecting two-story entrance pavilion. Throughout the nineteenth century, the three buildings were variously used as boardinghouse (by Henry Thoreau's aunts and grandmother), store, and residence until 1889, when the central and eastern sections were converted to a hotel, called the Thoreau House; the western section was added to the complex in 1897. With various additions in the area, the property continues as the Colonial Inn today.