I wonder who the people were who built it. I heard that few people lived in Oxford until the Middle Ages. The land where Oxford stands was all marshland before the Anglo Saxons drained it. I know that American Indians regard river confluences as holy places, so maybe the ancient Pagans who lived in Britain in those days were the same. Oxford of course is beside a major river confluence. When we were "civilized" by the church and state, all that ancient wonder was lost.
I'll have to take a stroll down to St Johns and see if they'll let me get a closer look. I'll try to get some photoes!