6: Keystone

If you stand on the star and look up to the top of the 2nd gable you will see the keystone

A keystone is the wedge-shaped stone at the apex of a masonry arch or typically round-shaped one at the apex of a vault. In both cases it is the final piece placed during construction and locks all the stones into position, allowing the arch or vault to bear weight.  The St Mark’s keystone has the date - 1853