The tower is placed just where we should expect to find that the need of fortification was felt. South and East, Oxford was now protected by the Thames and the Cherwell as well as by her "vallum," and on the west was the Castle. But the North Gate needed protection, and D'Oigli built the tower of S. Michael's to give it, spiritual and temporal both. At a later date there was erected a chapel, also dedicated to S. Michael, near the South Gate, and with reference to this church and chapel and the Churches of S. Peter in the East and in the West, there is a mediæval couplet which runs as follows: "Invigilat portæ australi boreæque Michael, Exortum solem Petrus regit atque cadentem." "At North Gate and at South Gate too S. Michael guards the way, While o'er the East and o'er the West S. Peter holds his sway." The military character of S. Michael's tower is marked by that round-headed doorway, which you may perceive some thirty feet from the ground on the north side. Just as the blocked-up archways at the top of the Castle tower once gave access to the wooden galleries which projected from the wall, so this doorway opened on to a lower gallery which guarded the approach to the adjoining gateway. On the south side of the tower you will find traces of another doorway, the base of which was about twelve feet from the level of the ground. It is reasonable to suppose that the tower projected from the north side of the rampart, and that this doorway was the means of communication between them. The other doorway, on the west side, level with the street, gave access from the road to the basement story of the tower. Architecturally the tower may be said to be a connecting link between the romanesque and Norman styles. The system of rubble, with long-and-short work at the angles, has not yet given place to that of surface ashlar masonry throughout, and the eight pilaster windows, it should be observed, of rude stone-work carved with the axe, present the plain, pierced arches, with mid-wall shafts, which preceded the splayed Norman window and arches with orders duly recessed. The church itself adjoining the tower is of various periods, chiefly fourteenth century. It was, together with S. Mildred's, united (in 1429) to All Saint's Church, which then was made a collegiate parish church by the foundation of Lincoln College adjoining. Not only was Robert D'Oigli a builder of walls and towers, but, in the end, of churches also. The Chronicle of Abingdon Abbey records the story of his conversion.