Ken Follett's The Pillars of the Earth

First you need to find the keys. To do this go to the cemetery behind the cathedral.
There find the novices that are fishing. Overhear them talking about how thy stole the keys and where thy hid them. To find them go inside the cathedral walk to the very end of the building. Inside the crack in the wall you will find the keys. Remember to use the hint you got from overhearing the novices talking on the crack. Now go outside the cathedral and outside the courtyard to the building by the fields. Use the keys on that building and you got the trophy.