I have been to Stonehenge many times and am a member of English Heritage as is this care taker.
While the circle may have continued around the full 360 degrees to some extent, I doubt that it was at all one level.
This is a picture from one such visit that I took standing roughly adjacent to the "heel stone." Notice the height of the stone just to the left of the picture center (ignore the stone in the "dead center" because that one is in the distance and actually at the far end of the circle from where this picture is taken.)
Notice the "tall" stone has a receiving stabilizing "key" in which a stone would likely have been intended to fit, but this stone stands ~ 3 - 4 feet higher than the stones on either side of it. Look in particular at the "short" stone to the left of the "tall" stone (not the one with the stone atop it pointing away -- that is within the center of the ring), and notice the distinct difference in height.
FReegards!
