Wow, I haven't logged onto this forum in well over three years... Glad I finally remembered my password

Anyway; I've had this game a while and actually finished it with pretty high levels though I cannot for the life of me find that Barheim Key. I know where the door is that needs the key but I can't seem to find it. Do I earn it in a side-quest or do I have to walk around constantly mashing the X button?

Replies, please and thank you!