It appears that the Dwarves are a bit paranoid, as these fortifications guarding their realm seem nigh impregnable and have even succeeded in keeping us out. Their secrets and technology will remain a mystery for now. There must be another way in, because it defies common sense that so many endothermic creatures can live off the oxygen from one single, albeit large, entrance.
"Sometimes I think we are alone in the universe, and sometimes I think we are not."