North Meadow is a 23-acre recreational field located around 97th street, north of the Reservoir, south of North Woods and between the park drives. During the Revolutionary War, this field, along with the Great Hill to the west and East Meadow to the east were the location of camps for British and Hessian troops who were occupying the forts on the bluffs just to the north to seal off lower Manhattan from the colonial armies. The area was listed as a "Play-Ground" on the 1868 Greensward Plan for the park and ballfields first appeared there in the 1870s, despite long-standing and ongoing disputes between the upper class (that saw the park as pastoral art) and everybody else (who saw it as a place for recreation).
The convention of notating closed greens with Red flags dates from the 1880s, when there were additional flags indicating suitability for specific sports - blue flag with a white ball for baseball, white flag with a red ball for croquet, blue pennant with white stars when open to the public in general. (Rosenzweig pp 314)