Something that didn't make sense
On all the maps I have seen, it seems as if Mordor has a back door, where there is no mountains. It seems to head into Rhun and Khand.
Now, in the movie version of Two Towers, when Frodo and Sam are near the Black Gate, it is quite obvious that the company of Easterlings they see is marching from the South. Now, if this back door into Mordor is there, couldn't they have just marched through that? And even if there isn't one, wouldn't it make more sense for the Easterlings to come from the North, after coming out of Rhovanion?