Sutcliff has to be one of the most consummately skilled authors in this genre, both for style and characterization. Her sense of time and place is wonderful, and she manages to powerfully evoke our interest in her character's struggles and triumphs. As another reviewer noted, Sutcliff's people are real, not some silly pasteboard stereotypes of modernity, flicked back about 1500 years.
The plot is tight, avoiding unnecessary haste (which helps to give a sense of reality, as well), but not degenerating into a slough of wasted pages devoted to trivialities. Sutcliff's keen sense of location is a delightful aspect of the story--- one feels that she was intimately acquainted with Great Britain's wilds, and loved them for what they are and were: solemn, unfathomable, and full of mystery.
An obvious scholar of Celtic and Roman traditions and culture, Sutcliff manages to subtly impart a great deal of information without lapsing into "textbookishness"--- that alone is no mean feat! Readers will find that there horizons have been broadened after diving into her books.
Sadly, most of her best fiction is out of print, but here are some titles of her most enjoyable stories-you might want to check the libraries:
Mark of the Horse Lord
The Eagle of the Ninth
The Silver Branch
The Lantern Bearers