It's really cool. The story revolves around this castle and it's inhabitants. The king has been taken hostage by a kingdom to the south, leaving his children in charge. Meanwhile, the fae realms to the north have become active again and are starting to agitate and move southwards. The world feels a lot like the northern part of GRR Martin's westeros, but not as sexual or aggressive as those characters. The lead characters in Shadowmarche are 15 year olds, after all.
It's well worth a read if you like Tad. I met him on this book tour, and found him to be a singularly fascinating dude. We hung out at Gencon Socal as well.
This book reads much faster than any of his other works, and the characters are very well done.