The abbey was built of Old Red sandstone, and belongs mostly to the end of the 12th and the beginning of the 13th centuries. The architecture is mixed, and the abbey is a beautiful example of the Norman and Transition styles. The total length is 235 ft., the nave being 133½ ft. long and 59½ ft. wide. The west front contains a great Norman porch and a fine wheel window. The nave, on each side, has nine pointed arches in the basement storey, nine round arches in the triforium, and thirty-six pointed arches in the clerestory, through which an arcade is carried on both sides. The tower, at the intersection of the nave and transepts, is of unusually massive proportions, being 30 ft. square and fully 100 ft. high; the network baluster round the top is modern. With the exception of the north piers and a small portion of the wall above, which are Norman, the tower dates from the end of the 15th century. The whole of the south transept has perished. The north transept, with early Decorated windows, has been covered in and walled off, and is the burial-ground of the Kerrs of Fernihirst, ancestors of the marquess of Lothian. The earliest tombstone is dated 1524; one of the latest is the recumbent effigy, by G. F. Watts, R.A., of the 8th marquess of Lothian (1832-1870). All that is left of the choir, which contains some very early Norman work, is two bays with three tiers on each side, corresponding to the design of the nave. It is supposed that the aisle, with Decorated window and groined roof, south of the chancel, formed the grammar school (removed from the abbey in 1751) in which Samuel Rutherford (1600-1661), principal of St Mary's College, St Andrews, and James Thomson, author of The _Seasons_, were educated. The door leading from the south aisle into a herbaceous garden, formerly the cloister, is an exquisite copy of one which had become greatly decayed. It was designed by Sir Rowand Anderson, under whose superintendence restoration in the abbey was carried out. The castle stood on high ground at the south end of the burgh, or "town-head." Erected by David I., it was one of the strongholds ceded to England in 1174, under the treaty of Falaise, for the ransom of William the Lion. It was, however, so often captured by the English that it became a menace rather than a protection, and the townsfolk demolished it in 1409. It had occasionally been used as a royal residence, and was the scene, in November 1285, of the revels held in celebration of the marriage (solemnized in the abbey) of Alexander III. to Joleta, or Yolande, daughter of the count of Dreux. The site was occupied in 1823 by the county prison, now known as the castle, a castellated structure which gradually fell into disuse and was acquired by the corporation in 1890. A house exists in Backgate in which Mary Queen of Scots resided in 1566, and one in Castlegate which Prince Charles Edward occupied in 1745.