I am giving this book a 2-star rating just for my respect for the author's vast encyclopedia-like knowledge of medieval history. Also, his previous book, 'The Name of the Rose' was a good read, though also rather dense for the average reader. Otherwise, this book deserves a 1-star.
It was a chore to stay engaged with the story. The sheer volume of history unloaded in bursts one after the other left me overwhelmed. It was difficult to recall what happened 5 pages prior to where you were at any point throughout the book. Still, I forced myself to keep reading chapter after arcane chapter each of which kept throwing more dense historical discussions between the main protagonists.
The plot itself is unrealistic. The notion that a group of friends can sustain this level of interest in this so-called 'Plan' over a period of months and years, is difficult to imagine. You'd think that with their level of knowledge and deep discussions on various historical events, they'd rather spend their time in academics in some university, instead of wasting their time on a fictitious Plan.
I read 'The Name of the Rose' a second time, just to refresh my memory. I might even go back for a third time, maybe after a few years. This one - I will never attempt reading again.