http://beyond.divinedivinity.net/help/act1_citadel03.htm

if you missed the BF key one in the starting level =>
you're probably having trouble at number 8, right? Ok, go to 11 and survive the illusion there. You will be teleported back into a small room, then you're in this maze:

http://beyond.divinedivinity.net/help/act1_citadel05.htm

proceed north and you will access a few small rooms. One has a large book on a stone block and there you will see the next BF 1 key. I'd leave the gargoyles and after you got the key I mentioned - you can return and kill them