In 1768, the British entered Boston Harbor, which began an eight year occupation of the city. Two years later the Boston Massacre took place on the streets of Boston, angering Patriots all the more.
In March 1776, the British evacuated the city of Boston. Washington had placed fortifications and cannon on Dorchester Heights, which from the south, has a superior position above the city.
Canon had been retrieved from Fort Ticonderoga.
On March 17, the British retreated with 11,000 British troops and some 1,000 Loyalists, departing by ship.
More information can be found at History.com.