Rambuchan:
What a terrific telling of a tale of bold adventure and heroic challenges.

Methinks the great Taliesin's reputation is in danger of being overshadowed. Many of the various tellings of Arthur's legend portray him as the last hope of a Christian, Romanized Britain, being invaded by the pagan Saxons and their evil ilk. I like your version. Arthur was, after all, a Celt. Your telling reminds me somewhat of Marion Bradley's "The Mists Of Avalon". Thank you for your contribution to this thread. Your voice is one of the clearest and most thoughtful heard in these halls of CivFanaticism.
Thank you also for the very gratifying compliments about the art work, 'pedia & music; they definitely make the game. I debated long with myself about the extra-Celtic type music (there's some South American stuff too), but I do like the way it sounds, so in the end it got in. Wait 'til you hear one of the 3rd age pieces; it rocks.

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To address your question of gov't types, that is one I have not answered myself with much satisfaction. There are 3 gov'ts available at the close of the 1st age, the Age of Tribes: Druidic Circle, The Great Thane, The Witan.
Druidic Circle is meant to mimic what it would have been like for a tribe/civilization/culture to have been ruled by a religious oligarchy, a priest class. Corruption is not too bad, but unit support is not great (better in smaller settlements) and is somewhat expensive; a reflection of how the warrior class is diminished in a cleric led society. Because things religious take precedence, production cannot be hurried.
With the Great Thane you are the ruler, but not absolute. You are beholden to your vassals, who are semi-autonomous and powerful in their own right. With this system, the people are more oppressed; production is hurried at the expense of the citizenry. Corruption is similar to Druidic Circle, but there is better unit support and it is less expensive. However, with large cities, metropolis sized, unit support is greatly diminished; your vassal lords have a much larger labor pool and tax base, so are more independent, less reliable.
The Witan attempts to sort of synthesize the previous two. It is like a council that advises the leader. Whether or not the advice is heeded, depends largely on the relative strengths of the leader versus the council. Since the people feel better represented, there is less corruption than Druidic Circle or The Great Thane. Additionally, with a greater sense of general well-being, the people are more productive, allowing a professional military class to develop. Consequently, there is greater unit support at the metropolis level, but if it is exceeded, the soldiers are more expensive. Furthermore, the govt must pay to have its citizens hurry their productive efforts.
So, there is the low down of the governmental situation. As you can see, there are trade-offs. It is difficult to advise which is the best. It all depends on the ratio of towns to cities to metros, and how much expense your treasury can sustain. From the way you describe your city size, I am inclined to suggest either The Great Thane, or The Witan.
Hope this helps.
I greatly look forward to your next installation of King Arthurs exploits and the tales of mighty
CAMELOT!