Looking at the old photographs that have been posted I'd go with Reg's identification of the structure in the centre of Ar Rutbah as being the old fort.
The triangular structure visible on one side of the fort in the photographs that have been posted has gone, but the shape and location of the structure relative to the curve of the wadi directly to the north are consistent with what is visible in Forget's photograph which seems to have been taken from a position to the west-south-west of the fort.
The other photographs that have been posted all appear to have be taken from a position to the north of the fort, but without the curve of the wadi being visible in the foreground.