From its founding in 563 A.D. by St Columba, Iona became the most significant ecclesiastical centre in Scotland. Its power grew with the Scotti until, ironically, the triumph of MacAlpine ultimately led to the shifting of civil and ecclesiastical power out of Argyll, increasingly ravaged by Norse invasions. The eventual adoption of Roman Catholicism by Queen Margaret ended the pre-eminence of the Celtic Church and of Argyll in ecclesiastical influence.
If the Norse who raided had dealt serious blows to church and state in Argyll, those Norsemen who had settled helped to mould a new local polity; firstly as an effective extension of the Norwegian Kingdom, but latterly culminating in the creation of the Lordship of the Isles under Somerled. There are physical remains since, by the late 12th century, the building of stone castles began on Argyll's seaboard. Suibhne, Lord of Knapdale and evidently of Norse descent, built Castle Sween in the late 1100s. Rothesay Castle, on the other hand, dating from around the same period, was built for the Stewarts, agents of the Scottish Crown. The appearance of these fortified masonry enclosures in Argyll emphasises that the local economy was sufficiently vibrant to support them and that opposing spheres of influence made Argyll a land in which territory and control of communications had to be held from strong bases. The positioning of the castles makes it clear that the sea and sea lochs were not barriers but seaways. Dugald, son of Suibhne, held the castle of Skipness in 1261. The wider Norwegian cause was not fortunate : although in 1230 and 1263, the Norse captured Rothesay Castle, the later campaign culminated in the decisive Norwegian defeat at the Battle of Largs.
The Lordship of the Isles
The 12th century had in fact produced, from the fusion of Gaelic and Norse culture, its own political power base. Somerled and his MacDougall and MacDonald successors to the Lordship of the Isles became as much a threat to the Scottish Crown as Somerled had been to Norwegian influence. Castles proliferated throughout the Medieval Period as did clan groups. Both benefited from shifts in power and influence. Some castles (such as Tarbert) owe their 13th and 14th century evolution to extensions of Royal power; others such as Dunstaffnage (founded by the MacDougall Lords of Lorn in the 13th century), were power-bases for the dominant local power.