Following on Phil's comments, there's a short entry on the NYS Division of
Military and Naval Affairs site:
www.dmna.state.ny.us/forts/forts5.html

This states the post was abandoned in 1764. There appears to have been some
traces left by 1777, or at least memory of the location of the blockhouse,
as witnessed by this entry in the Colbraith journal:

Aug 1st - Three Oneida Indians came express from their castle informing us
that they
 had seen three strange Indians, who told them that there were 100 more at
the Royal
 Blockhouse, and that they were to march for this place.

"New York's forts in the Revolution" by Robert B. Roberts, notes that
construction was authorized by Sir Jeffrey Amherst, probably in 1759, in
conjunction with the construction of Forts Brewerton and Bradstreet(Oswego
Falls).

regards,
Greg


Greg Ketcham
webmaster,
"Drums Along The Mohawk: the American Revolution on the New York Frontier"
http://www.nyhistory.net/~drums