The top left, there's a tablet on the wall. The code is written on that.
Although, I most likely will move them around now that I have more areas available. In addition to "buffing" the machines so they are worth it
Moving them would make it more fun. Though in my case it would give me a better chance at finding it since I somehow cannot get to it for the life of me.
I went to the top left of everything I could think of, whether it be the forest, the sewers, the castle, the town, even the other continent, but I can't see it.