Kate Mosse wrote Labyrinth as a paean to her beloved home from home, the medieval walled city of Carcassonne in southern France. At times the love of the city gets in the way of the writing, especially in the amount of languid space devoted to the period around the attack on the city in the anti-Cathar crusade in 1209. For this is a book of two stories: a story set in 2005 about a secret organisation seeking to find important religious relics for their own nefarious ends and a story of how the events of the early 13th century tie into this 21st century plot. Eventually it turns into a decent conspiracy tale, but it takes a long time to get there as the 13th century story is what Mosse devotes most attention to. In the opening and closing sections of the novel these two story lines are well integrated, but in the long sagging middle the primary concern is the historical fiction element. As a novel it would have worked better if the historical sections have focused on what was needed to enhance the 21st century story, but instead it becomes so long-winded that it is hard to remember what has happened in 2005 by the time the closing section veers back to focus on that era.
There are two similarly name main protagonists: Alice a young English woman visiting southern France and Alais a young woman from the upper echelons of Carcassonne society. Those two characters stay in their own centuries, although the opening of the book suggested that it might become a time travel story. There is some level of connection between these two, but what precisely it is does not become clear until the denouement. The initial premise is that Alice is a volunteer on an archaeological dig in the Sabarthes Mountains, who chooses to do some unofficial digging and is somehow drawn towards the concealed entrance to a cave. This leads her to stumble into a grave and find a stone ring. At that point she loses consciousness and the mystery begins. The early chapters feed in parts of Alais' story, which works well until the 13th century narrative takes over and the 21st century becomes lost for much of the middle of the novel. It is a long book and not that well written, especially in relation to its stilted dialogue, and if a reader takes some time to complete the novel it will be difficult to keep the early details of Alice's story in their mind.
I brought a prior knowledge of 13th century context and that is what attracted me to the book, but I cannot avoid the conclusion that the overall novel would have been much improved with Alais' story being backgrounded and Alice's narrative taking the lead. What is beyond doubt is that the amount of historical information dumped on the reader at certain points was a drag on what could have been a much better novel.