Highly Descriptive, Somewhat Musical—and Super British
Mary Stewart knows how to turn a descriptive phrase. Check out this line about the creepy forest in Brittany, for example: "The air was full of the smell of damp and fungus and dead leaves and rich, rotting things" (II.8.2).
The cadence of the line is like the meter of a poem ("of damp and fungus and dead leaves…"). The language she uses here helps us see the situation as potentially fruitful (like the rich, rotting earth) but also full of danger (like the dead, damp, fungus-y bits).
Stewart also lets her Britishness show throughout the book, usually in Cadal's character, who speaks very informally and candidly with Merlin. Look at this moment, for example, when Cadal is reunited with Merlin after the whole terrifying Belasius incident: "High-handed little fool. Might have got yourself killed, meddling with that lot. […] You should've heard what I was calling you, too. Proper little nuisance was the least of it," (III.11.34, 36).
You know you've angered your British auntie when terms like "little fool" and "proper little nuisance" come out.