Published in 1951, The Armourer's House is one of Sutcliff's earliest novels. Set in the London of Henry VIII, it follows the "nearly nine" orphan Tamsyn, brought up by an uncle in Bideford in Devon, as she moves to London to live with another aunt and uncle (the armourer of the title). She has to settle in to life with a new family, even if they are entirely welcoming. She develops a crush on her oldest cousin Piers, who shares with her a love of ships and dreams of sea-going, but is apprenticed to follow his father as an armourer. And she gets to see the City of London and its surrounds.
That's pretty much the plot, which is by modern standards extraordinarily low-key, lacking any high drama or tension at all. But that very restraint makes its little ups and downs unexpectedly effective, and it finds motive power in other ways: most of the characters remain ciphers, but the central portrait of Tamsyn is compelling and her fears and concerns and hopes and dreams are what drive the story.
The chapters are largely independent episodes. The family goes down to market in Billingsgate, where Tamsyn is distracted by the ships. Three of the children encounter a wise-woman, who feeds them, grants Tamsyn a vague prophecy of future happiness, and gives them gifts. They celebrate Halloween. They see Henry VIII and Anne Boleyn going by in a barge, and cheer the queen when everyone else is cheering the king. And so forth. There is one surprise at the end, and a feel-good ending in which such plot threads as there are are happily resolved.
The Armourer's House also offers — with some quite long descriptions — a vivid portrait of the City of London in the early 16th century, when Deptford and Westminster were separate settlements and apprentices fought pitched battles with the Watch. Sutcliff includes quite long descriptions, and these are complemented by a dozen line-drawings by C. Walter Hodges.
"London still seemed a long way-off, and not quite real, with all its crowding towers and spires and pointed gables tinted lilac and tawny by the evening light; and suddenly, as the sun sank lower, it began to flash back from the glass-filled windows of churches and rich folks' houses, so that the City was a city in a dream, a city of golden windows."
Sutcliff deploys what is, by modern standards, unusually complex language for young children, using a broad vocabulary and long sentences (though mostly with run-on conjunctive constructions rather than nested subordinate ones). There are also some quite slow-moving passages. Presumably, given Tamsyn's age, The Armourer's House was aimed at children around eight, but I suspect only a few at that age will now have the patience for it.
The comfortable and secure family setting helps make Tamsyn's story accessible, but is not I think anachronistic: the Tudor period is perhaps the earliest in which something homologous to a modern middle class childhood existed. (Before then, families that had that kind of security and comfort were likely to be from a narrower elite.) Apparently Sutcliff herself described The Armourer's House as "a little too cozy and a little too sweet", but that's what makes it perfect as an introduction to her writing for younger readers who can appreciate her language and style but might find the plots of some of her other novels too confronting.
Note: the illustrations are, sadly, not included in the current paperback editions.