User blog comment:SayuriDarling/The House of Hades Official Discussion/@comment-10137020-20131009195220/@comment-3146930-20131009200115

The Doors of Death were moved to Tartarus by Gaea so she could use it to raise monsters and souls. The chains in the cover are holding the doors in place, preventing them from moving as they normally would. When Thanatos is using them, he doesn't need to use the button system.