They first prepared a "cheval de frise" to prevent British ships from sailing up the Hudson and outflanking the U.S. position. For more than a month, the troops transported boulders from the heights of Manhattan to the edge of the river, where they loaded them into a collection of hulks and cribs made of timber and stretched it across the river. When the cheval de frise was finished, they began work on the fort. Little soil covered the rocky surface, so men had to haul soil up from the low ground. They were unable to dig the customary ditches or trenches around the fort. The fort was built in the shape of a pentagon with five bastions. The main walls were made of earth, constructed with ravelins with openings for guns from every angle. The fort enclosed a total of three to four acres. The troops built an abbattis around the fort. After the barracks were finished in September, all the troops in the area were placed under the command of Major General William Heath. Washington established his headquarters near the fort.