Thanks for this great post.
I grew up in the 60s and 70s on Arlington Avenue with the abandoned Nipnichsen tennis club as an extended backyard.
From the looks of it, Fort No 2 was 50 yards from our backyard.
If only we had known back then.
“The Nip” was an anything goes kind of place. The owners showed little interest, and the city couldn’t even track them down when it decided to bulldoze down the fire damaged clubhouse in 1973.
Kids in the area dug holes, set fires, built forts, set off fireworks. No one (other than parents) seemed to care. There was a wild west feel to the place.
It would have been easy to just start digging and see what turned up. Now it is likely too late.
I posted a history of The Nip, from 1910-2000 on the FB RIVERDALE NEW YORK MEMORIES page, but did not delve at all into the Fort #2 aspect.
Here’s a version of a diagram I made for the FB post using a 1951 aerial view. I’ve circled the spot you describe as the location of the fort.