Stroud's The Last Siege

I really thought this hard-to-find novel by the author of The Bartimaeus Trilogy would be fantasy. I thought, reasonably enough, that it was fantasy. There's the fuzzy castle on the cover, and the cataloging information states "A chance encounter on the snowy slopes of a castle moat throws together three lonely teenagers, Emily, Simon, and the highly imaginative but enigmatic Marcus, who is the catalyst for all that follows."

What follows is that the children get kicked out of the castle ruins by the caretaker and decide to spend the night there. This involves fibbing to parents about where they are, and lots of chases about the place, some involving suspected ghosts. This is more of a survival tale, so I will try it out on those fans. That, and any child who might like to spend some time hanging out in castle ruins, pretending to be ancient kings defending their stronghold. I would have loved it.