Originally Posted by autoxinvr6
I figured it out. If you go to the end of the tunnel in the ruins to the rusty door there is a notch in the wall to the right where there is a secret passage. Follow that to the chest behind the door for the key.
Good job! The game content is not very dense btw. It's too easy to finish it when opting to not collect everything. I got about 95% of all the items though x_x too lazy to find the rest.