The Genbu Castle is the a fortress built by Dark Lord Xavier's Apostle, Genbu. The castle has a blocking magic that locks those inside and makes so that it always must have at least one human being trapped within its walls. In order words, it's impossible to escape without leaving someone behind. The only way for a person who was left behind to escape is for Genbu to remove them himself.
The castle is a special place trapped between dimensions where time does not pass, which means one could be trapped for eternity without aging. Some characters in the Rance world have been trapped there, and thus are physically younger than they actually are, like Bird Lithfie or Rizna Lanfbitt. The dimension is unique and follows its own rules, which can make strange abnormalities like the conceiving of a human devil half-breed.