The eastern termination of Carileph's choir had been apsidal; it was found to be in a very unsafe condition, cracks and fissures appearing in the walls. Various bishops and priors sent aid towards "the new work," but actual building did not commence until after the death of Bishop Poore in 1237. The erection was commenced by Prior Melsanby and, of course, necessitated the taking down of Carileph's apses. The revaulting of the choir was undertaken at this time, doubtless, for artistic reasons, to bring the new work into harmony with the old. The Chapel of the Nine Altars is a rare and valuable specimen of Early English Gothic architecture of remarkable and graceful design. Below each of its nine lancet windows was originally an altar, dedicated to different saints. Its great height was obtained by lowering the floor, so that the unity of the whole exterior should not be destroyed. Prior Melsanby is also said to have put a new roof on the church. Prior Hugh de Derlington, who was at the head of affairs from 1258 to 1272, and later from 1285 to 1289, added a belfry to the central tower. John Fossor, made prior of Durham in 1342, inserted the large window in the north transept and the west windows of the nave. Bishop Thomas de Hatfield (1346 to 1381) seems to have done no architectural work beyond the erection of his own throne and tomb (in which he was afterwards buried) on the south side of the choir. This is an elaborate and sumptuous piece of work, and shows remains of rich colouring and gilding. About this time, also, the beautiful altar screen known as the Neville screen was erected. Its cost was principally borne by Lord John Neville, though the Priors Fossor and Berrington and the subordinate cells of Jarrow and Monkwearmouth were also contributors. The screen is of stone--very light and graceful, and originally contained in the niches 107 figures, which have unfortunately been destroyed. Bishop Walter de Skirlaw, who occupied the episcopal throne from 1388 to 1405, was a great builder. To him mainly we owe the present cloisters, though they were completed by his successor, Cardinal Langley, in 1418. The monks' dormitory on the western side of the cloister is also of this time. On the southern side was the refectory. This portion was rebuilt by Dean Sudbury between 1661 and 1684 and converted into a library, and such it remains to-day. Near the refectory is the kitchen, built by Prior Fossor. It is octagonal in plan, and possesses a fine groined roof. It is now attached to the deanery, and known as the dean's kitchen. We must now turn our attention to the erection of the present central tower. The belfry added by Hugh de Derlington was in 1429 struck by lightning and set on fire. It must after this have been repaired in some way, but in 1456 it was in a very unsafe and dangerous condition, as the following letter written by the prior, William Ebchester, to Bishop Neville testifies:--