Mud and shells:
Fort Mifflin was built on the aptly named “Mud Island.” Thomas Paine, the eloquent pamphleteer of the Revolution, personally made a short visit to Fort Mifflin in September and observed that the muddy conditions of the island were, at the outset, an advantage. In a letter to Benjamin Franklin, Paine made this explanation:
At Noon I went with Col. Greene, who commanded at Red Bank, over to fort Mifflin (Mud Island) the Enemy opened that day 2. two Gun Batteries and a Mortar Battery on the fort. They threw about 30 Shells into it that afternoon without doing any damage; the Ground being damp and Spongy not above five or Six Burst, not a Man was killed or wounded.
https://founders.archives.gov/documents/Franklin/01-26-02-0421
Joseph Martin has a similar explanation:
In front of the barracks and other necessary places, were parades and walks, the rest of the ground was soft mud. I have seen the enemy’s shells fall upon it and sink so low that their report could not be heard when they burst, and I could only feel a tremulous motion of the earth at the time. At other times, when they burst near the surface of the ground, they would throw the mud fifty feet in the air.
https://play.google.com/books/reader?id=ZbdcAAAAcAAJ&pg=GBS.PA62&hl=en
The British eventually figured this out and, by shortening the fuses of explosive shells, were able to be more destructive.