Far and away the most stunning thing I saw when I visited London as a tourist in 1979 was the wall around The City of London. This is the ancient wall that at one point protected the city from attack. Why was it stunning? Because we walk up to the wall and look down a deep ditch of about twenty(?) feet to view the top of the wall. From the go go American point of view where things are transient and everything is new, to realize I was standing on the results of two thousand plus(?) years of human habitation, which had slowly raised the level of the earth all around that much, gave me a visceral sense of my connection with human history.