The Round Church in Cambridge is probably one of the oldest surviving buildings in the city.
Dating from 1130AD, this is (according to the website) one of only 4 circular churches in England. It is now maintained by Christian Heritage. Not the easiest building to take a picture of, sitting as this is on a busy crossroad in the city. A lovely little church, well worth the visit for sheer character and history. Please click-through on the picture for a larger version.