That castle thingy has unreasonably deep lore for being irrelevant to the plot.
Basically, a longgggg time ago, the human universe came into existence, which the Gardeners didn't like, cus we where kinda screwing things up (choking out the other universes). So they tried to destroy the universe, by sending one of their own in (the eye I'm the sky), the universe didn't like this very much, so it made magic. The first 4 mages were aligned to: Metal+ Fire, Metal+ Water, Wood+ Earth, Wood+ Air. For a while, they all fought the intruder, until they learned about the truth of the universe. Two of them, it shocked so much they left. (The wood mages, who later got married.) One of them, it drove into a nihilistic rage (Metal+ Water, who was the corpse lady in the basement), and one was driven to desperation. (Metal+ Fire, the lord of the house)
Corpse lady betrayed everyone, and started working with the Gardeners, so her husband (Lord of the house), decided he had to put an end to it, and too his wife. He captured her, and tried to kill her, but couldn't bring himself to do it. So then he sat in his house while the world was destroyed, the universe literally burning down around him, until all that remained was his house, and a few distant remnants (the wood people). As the Interloper approached, he felt resolved, and Knew what he had to do. He only wished his daughter didn't have to die with him.
So he set a trap, and captured himself, the interloper, his wife, and his daughter into a pocket dimension that's constantly being destroyed and created. If he wavers for even a moment, it will collapse, and his millenia of suffering will be for nothing.
As for the green lord thing. Honestly no idea what my motives where.
Wish I knew LOL