I presumed the rest of the cairn up there was on the same level as the tower, just before the high point drops down to the rest of the moor? I guess we are talking about the same old pile of stones.
Probably the same pile of stones - a round embankment 6 or 7 metres across - could be the kerb of a ring cairn. No photographs of it as there were a lot of people sitting on it while I was there.