I can change this around if need be, but to answer the questions in reference to Ferron as they are in my head : 

I think Ferron is a middle-elder god, being born before human's propagation and Hayim and Iva, but after Agamemnon, Cimeru, and Oao. He is probably also younger than Ragnanodden and Sveiland. I am not sure if he was born before or after Anu, I will that up to Zee. 

And for whether Deep Castle is the mortal world, or its own world. I think the first two layers (The dirt hovel and stone door) are physical things within the mortal world, but everything below is within its own plane of existence.