JJ Annaud explains (2 disc special edition) how he went to work and look for suitable places to film the monastery. He must have been inspired by the Cathar castles in the south of France. If anyone is interested here's a link to get an idea of what they look like.
http://en.wikipedia.org/wiki/Cathar_castles
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It has nothing to do with the look of a cathar castle. The look and shape of the scriptorium-tower in the movie was taken from the description in the book. It has that particular shape because of the library. The library in the book wasn't a random labyrinth but had a very organised system to it. The letters above each door in each room in the library, form a word together that describes a region in the (then known) world, and all the books from that region were stocked there. Very ingenious. That's my favourite part in the book: discovering there was actually a system behind the labyrinth. Too bad that that was lost in the movie. In the movie it just seems like any random labyrinth.
shareAccording to the description in the book the labyrinth / library is flat; just one level. J.J. Annaud mentions this explicitly during his comment (on the DVD) when he explains how he developed his view of the movie's scenery.
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