There are countless retellings and adaptations concerning the life and times of King Arthur and the Knights of the Round Table, and I'm not even close to having read all of them. Therefore, it's impossible for me to say that Rosemary Sutcliffe's version is the definitive Arthurian retelling. However, it's certainly one of the best. Told in Sutcliffe's graceful prose that is both epic and intimate when need-be, and the tricky subjects like incest, adultery and bloodshed are conveyed without being either too prudish or overly graphic.
This first installment in her "King Arthur trilogy" is thicker than the next two books combined, and Sutcliffe draws on a wide range of sources with which to build her own narrative. Beginning with Merlin's boyhood and his activities at Tintagel, Sutcliffe goes on to the circumstances of Arthur's birth as outlined in Geoffrey Monmouth's The History of the Kings of Britain, giving us her account of his birth, fosterage, and eventual crowning when only just fifteen.
From Thomas Malory's Le Morte D'Arthur she brings in the love triangle between Arthur, Lancelot and Guenever, Arthur's struggle to establish peace in Britain, and the forming of the Knights of the Round Table.
From this point, Sutcliffe moves into several other stories concerning the Knights of the Round Table, including Sir Gawain and the Green Knight (the most famous translation by Tolkien: Sir Gawain and the Green Knight, Pearl, Sir Orfeo) the Kitchen Knight (also one of my favourite picture books by Trina Schart Hyman: The Kitchen Knight: A Tale of King Arthur) and perhaps the best rendering of the tale of Tristan and Iseult there is (it almost deserves to be its own book).
Merlin and Morgan le Fey drop out of the story surprisingly quickly, and most of it is concerned with knight's errands and love stories, most of which can be read out of order, for this is not a novel so much as it is a compilation of stories. It can be rather difficult at times to keep track of all the interwoven stories that double-back and twist about and get steadily more complex as each chapter goes on.
"The Sword and the Circle" is followed by two sequels, "The Light Beyond the Forest," which recounts the search for the Holy Grail, and "The Road to Camlann" which concludes the trilogy with the destruction of Camelot and the disbandment of the Knights of the Round Table. I'd recommend tracking down the three-in-one version: King Arthur Stories.
The amount of story that Sutcliffe is trying to get across means that characterization beyond broad brushstrokes is minimum, and often motivation is completely lost, but what she still manages to skillfully convey the depth of human emotion that is so prevalent in these legends: the longing for the divine, the pain of love, and the mindlessness of hate. Arthur is perfectly portrayed as a man who rises to status of beloved ruler not through physical prowess, but his strength of leadership and his ability to create peaceful resolutions. Yet I got the sense that Sutcliffe was more interested in Lancelot, what with his twisted face and passionate heart. There are probably more pages dedicated to him than any other character in the entire trilogy.
She also crafts the unforgettable images of Arthurian legend that seem to be known to everyone: the sword in the anvil in the churchyard, the white hand in the lake clutching Excalibur, the hall of the Round Table, the byre of Elaine floating down the river outside Camelot, Merlin sleeping under the Hawthorn tree...the list goes on, and all of it is encapsulated in her rendering of medieval Britain: the dark forests and cool lakes, standing stones and mysterious wells, castles and hermitages.
Sutcliffe has created a very "pure" vision of the Arthurian story, in comparison the revisionist treatments that many authors use on the legends today (usually by giving them a feminist slant). But here we have a sense of the original story, much like the retellings/compilations by Roger Lancelyn Green and Howard Pyle, in which the knights: "take oaths that always they would defend the right, that they would be the true servants and protectors of all women, and deal justly in all things with all men, that they would strive always for the good of the kingdom of Britain and the glory of the kingdom of Logres, and that they would keep faith with each other and with God."