User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-10137020-20130620135443/@comment-3146930-20130620200834

Considering the Doors of Death are Thanatos' personal way in and out of the Underworld, either he or Hades made the doors. However, considering one side of the Doors of Death are in Tartarus, it is possible Gaea moved the doors when Thanatos was captured. All he can do is slow down monsters and souls trying to escape using his doors