User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-6951062-20130312053730/@comment-3146930-20130312225542

The Doors of Death are the only known way out of Tartarus. However, Iapetus managed to escape Tartarus as well as an unknown number of Titans during the Percy Jackson and the Olympians series. There could be other ways out or maybe someone can only escape if Tartarus itself wants them to. Plus, if the Doors of Death have to be closed from one side, then it will no longer be a portal in/out of Tartarus and be useless to the person left behind.