Judging from the 4 golden tags, they probably summon males, from the other world, and lock them in the castle, and treat them as sex slaves to produce children, and probably keep the males that are born in there to be raised into sex slaves, while letting the female children free after a certain age of taking care of them, but they erase their memories after release, so they don't spread the existence of the men in the castle.
Guess number 2 is, they somehow reproduce with the Kall, and they have them in that castle, as POW's, and use them as sex slaves.
Just some blind guesses before I keep reading this. Probably/hopefully I'm wrong though (that's just fucked up if I'm right) because I don't think this manga could get this dark.