From what i understand from GT. The world he was in wasn't game world
It seem his fortress get teleported to other verse
Spoiler for General level of new world:
The general level of that world is quite low as level 40-50 Dead knight can fight evenly with the strongest fighter of a nation while his top summon is Primal Fire Elemental Level 80
(around 10m tall with appropriate fire aura)
Normal fighter need only 1 magic arrow to kill which is the lowest magic available to him
The spell he used in Prologue was 9th level spell in scale from 1 to 10 of the game. Needless to say that is OVERKILL!!!