User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-4555714-20130924001215/@comment-3146930-20130924002256

I doubt it. In The Lightning Thief, Percy, Annabeth, and Grover were able to just walk up to the edge of Tartarus without any kind of barrier. Then again in The Mark of Athena, there is just a large gaping hole in the ground that leads down to Tartarus.

The passage above seems to be describing a type of barrier all around Tartarus, but this is inaccurate from what we have seen so far and may just be something Rick decided not to mention. I also don't think Poseidon created a doorway for Thanatos to allow him quick access in and out of Hades' domain.