Other works of this century, not mentioned in the annals, are the entire removal of the lower stage of Norman windows in the aisles, these were replaced by wide windows of five lights each; the addition of a parapet to the apse; the erection of piscinas and other accompaniments to side altars, at the east ends of the choir aisles.

For the rest of the architectural history we have no chronicles to guide us, and are left to the stones themselves.  But there is very little difficulty in fixing at least approximate dates for all the later work.  The most important alteration in the fourteenth century was the removal of the stages above the four great arches of the central tower, and the substitution of a lighter lantern.  When this was done, the great round arches east and west of the tower were changed into pointed arches, but those north and south were left unaltered.  There is every probability that some signs of insecurity had made themselves evident.  We have seen that three stages of the Norman tower were erected by Abbot William of Waterville.  Though not so stated we infer from this that at least one more stage was afterwards added.  In any case the tower must have been a very massive structure, considerably higher than the present one.  In the early part of this century, in 1321, the great tower of Ely had fallen; and its fate may have warned the monks of Peterborough to see that the disaster was not repeated here.  This alteration must have been made, judging by the details of the architecture, in the second quarter of the century.  Above the lantern was a wooden octagon.  The views that are given of this hardly warrant the admiration that has been sometimes expressed, or the regrets that have been uttered at its removal.  It may have been designed to carry a wooden spire, such as was afterwards erected on the bell-tower.  But most will agree with the criticism that it was “a low and unsightly structure.”  It hardly rose more than eight or ten feet above the top of the lantern, and the whole height of the central tower, including the octagon, was less than the height of the south-western spire of the front.

To this century belongs the transformation of the triforium windows all through the nave and choir.  Parapets were at the same time added above the Norman corbel tables.  The change effected in the apse was the most noticeable; not only were the two upper tiers of Norman windows replaced by Decorated ones of larger size, but the three lowest ones in the centre were altogether removed, and their place taken by lofty archways, when the new building was built.  But we can judge of their appearance from the two side windows which still remain; these, being not now external, have had all the glass removed; but the mullions and tracery are perfect, and even the iron-bars across are still there.  At the inner surface of the wall the five lower windows have very good hanging tracery, of different designs.