Rosemary Sutcliff features, of course, in The Oxford Encyclopedia of Children’s Literature. Her entry concludes:
Critics of Sutcliff’s work sometimes comment on its difficulty both in terms of the language she employs and in terms of the historical depth her novels embrace. But for Sutcliff herself, these sorts of evaluations of her writing were welcomed as compliments. She prided herself on never writing down to her readers, expecting them instead to be enticed into enjoying a compelling and demanding tale by the pageantry of history and the warm humanity of people in every era. She carefully creates dialogue in her novels that recollects the speech of a bygone era without falling into what she termed “gadzookery.”Sutcliff also researched her novels with exquisite care, and they reflect her vast knowledge of military tactics, religious practices, landscapes, and the material conditions and artifacts of everyday life whether in a Bronze Age village or in a Roman legion on the move. Other commentators have noted the limited role that female characters play in her novels. Except for a few volumes that focus on a young woman, like Song for a Dark Queen (1978), which tells the tale of Boudicca, the queen of the Iceni who led a revolt against the Romans in A.D. 60, this is certainly true. Sutcliff often includes energetic and courageous women among her secondary characters, but providing insights into women’s roles in history is not among her greatest strengths.
Source: Megan Lynn Isaac “Sutcliff, Rosemary” The Oxford Encyclopedia of Children’s Literature. Ed. Jack Zipes. Oxford University Press, 2006. The Oxford Encyclopedia of Children’s Literature: (e-reference edition). Oxford University Press. Retrieved: 18 March 2012 http://www.oxford-childrensliterature.com/entry?entry=t204.e3114
ROSEMARY SUTCLIFF (1920-92) 
I love the word “Gadzookery”, and thought you might like the read the piece where Rosemary Sutcliff originally mentioned it.
“Victorian writers, and even those of a somewhat later date . . . saw nothing ludicrous in ‘Alas! fair youth, it grieves me to see thee in this plight. Would that I had the power to strike these fetters from thy tender limbs.’ Josephine Tey, whose death I shall never cease to lament, called this ‘Writing forsoothly.’ A slightly different variant is known in the trade as ‘Gadzookery.’ Nowadays this is out of fashion; and some writers go to the other extreme and make the people of Classical Greece or Mediaeval England speak modern colloquial English. This is perhaps nearer to the truth of the spirit, since the people in question would have spoken the modern colloquial tongue of their place and time. But, personally, I find it destroys the atmosphere when a young Norman Knight says to his Squire, ‘Shut up, Dickie, you’re getting too big for your boots.’ Myself, I try for a middle course, avoiding both Gadzookery and modern colloquialism; a frankly ‘made-up’ form that has the right sound to it, as Kipling did also. I try to catch the rhythm of a tongue, the tune that it plays on the ear, Welsh or Gaelic as opposed to Anglo-Saxon, the sensible workmanlike language which one feels the Latin of the ordinary Roman citizen would have translated into. It is extraordinary what can be done by the changing or transposing of a single word, or using perfectly usual one in a slightly unusual way: ‘I beg your pardon’ changed into ‘I ask your pardon.'”
–Rosemary Sutcliff: “History is People” (1971), published in Virginia Haviland’s “Children and Literature: Views and Reviews”.
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I think that readers have become used to improbably feisty, independent heroines in modern historical adventures, but the reality is that within their societies girls in the historical settings Rosemary Sutcliff used would not have had the freedom to act in ways that boys did.
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It’s quite hard to write a realistic action adventure with a girl as main character in a historical setting. Geoffrey Trease got round the problem by pairing a boy and girl together, of course.
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