By the time it was sketched by the military engineer Pierre Charles l’Enfant in around 1783, West Point had become the most strategically important fortress in North America. It had also evolved into a remarkably advanced system of layered defenses, with three concentric rings of 16 mutually-supporting enclosed positions and 10 major artillery batteries, all constructed to defend a 500-yard long Great Chain designed to stop a British invasion up the Hudson River. The innermost ring comprised batteries along both the west and east banks of the river and Fort Clinton positioned on the Plain above. The second ring contained four forts protecting the western and southern approaches on the west side of the river and three redoubts (small fortifications designed to shelter an infantry detachment) on Constitution Island defending the eastern approaches. The outermost ring combined four redoubts to add defensive depth on the western side and two redoubts to guard the landward approach toward Constitution Island on the eastern side of the river.