After filling the laboratory with things, he would need for his research into battleships, Kai Syaoran activated the many formations that covered the castle and even activated the time bubble that he had gotten his mother, the Forever General to set up for him before he left the underworld.
This time bubble manipulated time to his advantage. It created a time difference between the inside and outside of the castle.
A single day could go by in the real world and yet inside the castle, it would be much longer than a single day.
The time difference this time was far greater than when he was in the underworld.
Even Kai Syaoran felt that this time difference was terrifying. If he was not from an immortal race such as the Forever Devil race, he would never be willing to use this time bubble.
