In the early days of the American republic (and, perhaps, even in aboriginal days), the high bluffs in northern part of Central Park were the location of significant military activity and construction. As one of the highest points in Manhattan, the tops of these hills made it possible to see and attack invaders coming down the Hudson River, from north of the city and from Long Island.
Following a British attack on Stonington, Connecticut on August 9, 1814, during the War of 1812, General Joseph Swift organized volunteers to quickly build a chain of fortifications across Manhattan for the defense of Harlem Heights. Around 1,600 NY State militia spent the fall of 1814 in these fortifications, but the British never attacked New York City and the war officially ended in December of 1814. None of these fortifications ever saw combat and they were decommissioned and/or repurposed long before construction of Central Park began in 1858.
McGown's Pass was named for a tavern run by Scotswoman Catherine McGown and her children from 1759 into the 1840s. The tavern was located on what later became the Mount St. Vincent's Academy. The area to the north of these bluffs was a marshy area surrounding Harlem Creek which extended west to the Harlem River. Kingsbridge Road, the major artery linking New York City with mainland cities to the north, crossed Harlem Creek over King's Bridge and passed up through this area between two rocky outcrops before proceeding south into the city, which was then limited to lower Manhattan. Early nineteenth century illustrations (which can be viewed at the NYPL's online digital gallery) show a gate surrounded by forts atop the outcrops.
Control of the pass conferred military advantage. During the war for independence, British troops sealed off lower Manhattan by controlling the pass and camping on what are now the Great Hill, North Meadow and East Meadow. During the War of 1812 Fort Clinton and Nutter's Battery were built to guard the pass. (reference)
From the historical maps and what little information is available on the web, it is a bit difficult to determine exactly where McGowan's Pass was. The topography has been modified quite a bit since the 19th century. Harlem Creek has been filled in on both sides of what is now Harlem Mere and the eastern side of the marsh is now East Harlem around 106th Street. Kingsbridge Road is long gone and the eastern Park Drive now curves around the bluffs in switchbacks that are alot of fun to bike or skate down but a bit of a effort to get back up again. The area that had been cleared in military days to give full view of the surrounding countryside is now covered in lovely tall trees.
However, I will assume that what was the pass is now approximated by the modestly inclined path that rises from the midpoint of Harlem Mere, between Nutter's Battery and Fort Clinton and joins with the existing Park Drive to the south of where the park drive begins its switchback around Fort Fish.
Some contemporary maps are a bit indeterminate about the location of the pass and label McGowan's Pass as this valley between Fort Clinton on the north and Mt. St. Vincent on the south, which would lead it around the east side of the the Fort Clinton outcropping. However, the 1811 map given above lists it as clearly behind the fortifications.
Fort Clinton was named after DeWitt Clinton, Mayor of NYC during the War of 1812. Although no remnants of the fort remain, a monument was placed here in the early 20th century. That monument was extensively vandalized in the 1960s and awaited restoration at the time I visited in the summer of 2007.
Nutter's Battery was named after the owner of a nearby farm. I'm not sure what parts of the existing stone walls have any relationship to those of the fort.
Fort Fish was situated on the highest point in the area and heavily armed. It was named after the chairman of the City's Committee of Defense. It is now a secluded area just above the first switchback curve on the northern part of East Drive and it's fabled view of the surrounding area is obscured by foliage. The monument bench to park founding father Andrew Haswell Greene was relocated here when it's original location across the drive at the former St. Vincent's Academy location was converted to a composting operation in the 1980s.
Northwest corner of the park around 108th Street
The Blockhouse is the one remaining fortification of the four that were located in this area and it's speedy construction by volunteers is evidenced by the unusually rough stone work. The Blockhouse had a timber floor to support heavy canons and all four sides have two small gunports.
The upper two feet of the walls are different in composition from the lower walls and were probably added after the war when the building was used for storage. The stair and entrance are also not original. This rugged area was not considered suitable for park land and, therefore, not included in the original 1858 plan for Central Park, but was added in 1863 since the hills and swamp (now Harlem Mere) also proved unsuitable for commercial development. The remnants of the blockhouse were landscaped and treated as a picturesque ruin - a role that it continues to nobly serve. (reference)