LocationThe cut scene at the beginning of the game says that it is set in Aberystwyth in Wales. This place does exist and has a ruined castle which looks similar to the monastery in the game.
ReferencesKoudelka features a true historical figure: the medieval Franciscan monk and savant Roger Bacon makes his appearance in the monastery graveyard.
Mystic symbolsJust a small detail: one of the FMVs of the game (the one with strange symbols floating around, when the heroes talk about alchemy and mysticism) shows circles with Hebrew words engraved: Keter, Khokhma, and Bina. Those words (meaning Crown, Wisdom, and Understanding) are the attributes of God, according to the medieval Jewish mystical teaching, the Kabbala. They actually bothered to dig up 100% authentic and rare historical/occult material.
Information also contributed by festershinetop