4. "What is the Dark Tower?"
The Dark Tower appears to be a linchpin of time and space, it may connect alternate time lines or realities but its true nature (at this time) remains unclear. What is clear is that Roland's world may not be our own. There are similarities such as the song "Hey Jude" and other snippets of pop culture, but Roland never heard of a place like New York City and remains of lost technology are thousands of years old and yet hundreds of years ahead of our current technology. For more information on the Tower read the notes for "Insomnia" and "Black House" below (or if you don't want to read spoilers then go read the books!)
The revised edition of book 1 expands on this idea further with the whole "as above, so below" notion. Our planets, solar systems and galaxies may be nothing more than atoms in a blade of purple grass, while the atoms that make up our reality may in turn be other galaxies, solar systems and planets. What is constant, omnipresent and binding on all worlds is the Dark Tower.
Jake, Eddie & Odetta seem to have been drawn from "our" world based on the evidence that Eddie has seen the film version of "The Shining" as opposed to "hearing about that weird hotel..." (the latter statement of which would clearly place him in King's fictional universe of horror instead of a world grounded in reality.)