Not that I've got a clue about what I'm positing, but how would people been moving about in those days; which were the preferred routes other than the Ridgeway itself? Was there more traffic from a specific area to the west (Wales?)? Was it important that people saw it from far off? Goddess forbid, but could it be a huge advert for the biggest horse-traders in the country at the time? Was the whole of the Uffington complex the capital of a kingdom?
As for the third part, what about Dragon Hill making three? What if Dragon Hill was a gatehouse to the Castle, and people had to pass that, and somehow journey along the trail over the back of the horse to reach the Castle? Could that be akin to symbolically 'riding' the horse 'into' the castle?
I realise this is probably a tissue-thin raft of supposition, so I welcome any input!
treaclechops