During the night of March 4, 1776, American troops besieging Boston built two redoubts on these heights that made the harbor and town untenable for the British fleet and garrison. On March 17, the British fleet, carrying 11,000 effective men and 1,000 refugees, dropped down to Nantasket Roads, and from that point Boston was free. A strong British force had been expelled from one of the united American Colonies.