As early as 565 Celtic missionaries including Saint Columba had introduced Christianity into the islands. The exact date and the nationality of these missionaries are unknown, although it is recorded that Thuralf was sent as bishop in 1055. Uncertainty is the result of conflicting records as there was contention between Archbishops of Hamburg, York, and the Norwegians over ecclesiastical jurisdiction of the Orkneys. The diocese comprised the Orkney Islands off the north coast of Scotland. William, 1162-1168, was the first bishop with a fixed see at Kirkwall; the last pre-Reformation bishop was Robert Reed, 1541-1558. The cathedral at Kirkwall, erected 1138, in honor of Saint Magnus, whose remains it contains, is of strict Norman architecture relieved by a small amount of Gothic, and is at present used as a Presbyterian parish church. Close by is the Mass Tower dating from 1550, which was built by Bishop Reid.