I thought it was called Whitley Castle, although it is on the Maiden Way?
The imaging for that area is pretty poor, plus I don't know exactly where it is - never been there.
However, I would guess it's somewhere here:
http://maps.google.com/maps?ll=36.123283,-5.344130&spn=0.008444,0.010203&t=k&hl=en
Alston is just in the bottom left corner.