Last Siege by Jonathan Stroud (2003)

This actually was a pretty good young adult book, it was just that after reading Stroud's Bartimaeus trilogy, which was about magic, but with a fresh twist, I was expecting something more extraordinary. Three kids from today's England don't really belong with any group and are bored on Christmas break. They start hanging around a partially ruined castle near the town and with the help of Marcus' great imagination and knowledge of history, they start exploring the closed castle. It turns into a real siege at one point. The story is written from Emily's point of view, and it is interesting to observe the coolaboration between her and Simon and Marcus. It turns into a pretty touching story in the end.