The most impressive Iron Age structures of Orkney are the ruins of later round towers called "brochs" and their associated settlements such as the Broch of Burroughston and Broch of Gurness. The nature and origin of these buildings is a subject of ongoing debate. Other structures from this period include underground storehouses, and aisled roundhouses, the latter usually in association with earlier broch sites. During the Roman invasion of Britain the "King of Orkney" was one of 11 British leaders who is said to have submitted to the Emperor Claudius in AD 43 at Colchester. After the Agricolan fleet had come and gone, possibly anchoring at Shapinsay, direct Roman influence seems to have been limited to trade rather than conquest. However, Polemius Silvius wrote a list of Late Roman provinces, which Seeck appended to his edition of the Notitia Dignitatum. The list is famous because it names six provinces in Roman Britannia: the sixth is the dubious "Orcades provincia", of which recent researches re-evaluate the possibility of real existence. By the late Iron Age, Orkney was part of the Pictish kingdom, and although the archaeological remains from this period are less impressive there is every reason to suppose the fertile soils and rich seas of Orkney provided the Picts with a comfortable living. The Dalriadic Gaels began to influence the islands towards the close of the Pictish era, perhaps principally through the role of Celtic missionaries, as evidenced by several islands bearing the epithet "Papa" in commemoration of these preachers. However, before the Gaelic presence could establish itself the Picts were gradually dispossessed by the Norse from the late 8th century onwards. The nature of this transition is controversial, and theories range from peaceful integration to enslavement and genocide. It has been suggested that an assault by forces from Fortriu in 681 in which Orkney was "annihilated" may have led to a weakening of the local power base and helped the Norse come to prominence. Both Orkney and Shetland saw a significant influx of Norwegian settlers during the late 8th and early 9th centuries. Vikings made the islands the headquarters of their pirate expeditions carried out against Norway and the coasts of mainland Scotland. In response, Norwegian king Harald Fairhair (Harald Hårfagre) annexed the Northern Isles, comprising Orkney and Shetland, in 875.