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Westray is roughly 10.5 miles long and 6.5 miles at its widest part. It is composed mainly dark blue or grey flagstones belonging to the Rousay beds of the Middle Old Red Sandstone system covered with boulder clay and blown sand. The flagstones have provided the materials for the rubble walling of Noltland Castle, but the red freestone for the dressed work has been imported, probably from Eday. Timber for roofs and floors must have come from outside the island, probably from Norway, but the local flagstones, split thin, will have been used instead of slates.
Upon an island so devoid of natural resources, the large and powerful castle of Noltland seems out of scale. Its formidable defences, far in excess of anything similar in the Northern Isles, or indeed in Scotland, strike a note equally discordant. This great and strong castle, elaborately designed and executed with skill and lavishness, is certainly not to be explained merely as the manorial residence of a laird in Westray.
In The Baronial and Ecclesiastical Antiquities of Scotland, published in 1845-52 by the English architect R. W. Billings (with the assistance of David Balfour of Trenabie) he says:
“[the castle] was begun by Thomas de Tulloch, Bishop of Orkney, and governor of these islands under Eric of Denmark, 1422-28, and the initials TT., with the kneeling figure of a bishop, ornament the capital of the pillar supporting the great staircase.”