On the evening of 16 June, General Ward dispatched Col. William Prescott and about 1,200 troops from Massachusetts and Connecticut to occupy Bunker Hill on the Charleston peninsula. Upon reaching their destination, the chief American engineer, Col. Richard Gridley, recommended that the troops build their main fortification on Breed’s Hill, some 900 meters to the southeast, because it was more defensible. Prescott and part of the militia got to work constructing a small earthen fort and attached breastworks while other troops occupied the town of Charleston to its south. A few hours later, Maj. Gen. Israel Putnam arrived at Bunker Hill and took charge of the troops and a pair of 4-pounder cannon stationed there. Several hundred additional militia soldiers from Massachusetts and New Hampshire reinforced Putman on the morning of 17 June, bringing the total number of provincial militia on the Charleston Peninsula to around 1,500 soldiers.
"“Accordingly on the 16th of June, orders were issued that a party of about one thousand men should that evening march to Charlestown and entrench upon the hill. About 9 o’clock in the evening the detachment marched upon the design of Breed’s hill situated on the further part of the peninsula next to Boston, for by a mistake of orders this hill was marked out for the entrenchment instead of the other . . . it was nearly twelve o’clock before the work was entered upon, for the clocks in Boston were heard to strike about 10 minutes after the men first took their tools into their hands. The work was carried on in every animation and success so that by the dawn of the day they nearly completed a small redoubt about eight rods [about forty meters] square.”"
Rev. Peter Thacher, an eyewitness account written two weeks after the battleCommager and Morris, pp. 126-127.