Somerville occupied a conspicuous position during the entire Siege of Boston, which lasted nine months, and Prospect Hill became the central position of the Continental Army's chain of emplacements north of Boston. Its height and commanding view of Boston and the harbor had tremendous strategic value and the fortress became known as the "Citadel". Originally occupied by just 400 men, Prospect Hill became a primary encampment for American forces after General Israel Putnam's retreat from the Battle of Bunker Hill. It is believed that on January 1, 1776, the Grand Union Flag flew for the first time at the Citadel, the first official raising of an American flag.