Sec. 53. The position of the western tower in the plan is normally at the west end of the nave, with which it is connected by an arch, low at first, but loftier as time goes on, until, in later Gothic churches, its height frequently is nearly that of the whole nave. The remaining three walls are usually external, and clear of the aisles. But sometimes, owing to a freak of planning, or, more frequently, owing to the conditions of the site, the tower is, as at Bibury, at the west end of one of the aisles. At Gedling in Nottinghamshire the tower and spire are at the end of the north aisle. The tower of St Michael's, Cambridge, is at the west end of the south aisle: probably the western extension of the church was prevented by the neighbourhood of the street, a circumstance which often accounts for the irregularity of plan in some town churches. At St Mary Redcliffe, Bristol, built on the edge of the "red cliff" from which it takes its name, the tower and spire are at the end of the north aisle: had they been planned in the usual place, a full bay of the nave would have been sacrificed. The tower at Spalding was planned, in the first instance, to stand against the south wall of the west bay of the south aisle: subsequently a new south aisle was built east of it. One of the most curious instances is that of St Mary's at Leicester, where the tower, subsequently, as at Spalding, heightened by a spire, was planned in the thirteenth century, outside a very narrow south aisle. A tower at the west end of the nave would have encroached upon the inner ward of the adjacent castle. The chancel of St Mary's was used for collegiate services, and parochial accommodation was limited. Towards the end of the thirteenth century, a very wide south aisle, a parish church in itself, was built the full length of the nave, and overlapping the chancel at the east end. The tower was left standing on piers entirely within the west end of the new aisle. It may be added that, where towers occur at the end of aisles, they seldom project beyond the west wall of the nave, but open into the nave by an arch in the north or south wall, as the case may be. Plans with two western towers, as at Melbourne or St Margaret's at Lynn, are of very rare occurrence; and, where they are found, the plan was probably designed on more ambitious lines than those of the ordinary parish church.