The world as we know it is only one of many. Every type of Fable has a different and unique Homeland based on the culture that created it (Germanic, Indian, Russian, etc). Their Homeland reflects the root culture and is entire unto itself. The only way that someone can travel between these separate worlds is by using magical gateways that link the worlds together. 
When the Adversary began his bid at domination, it was through these gateways that he sent his unstoppable armies. He jealously guarded them, knowing their strategic importance. The Adversary destroyed many of the gateways in an attempt to keep them under tighter scrutiny, and, conversely, the Fables also fiercely guarded the gateways to the Mundane world to keep their enemy out of their new home. 

However, the Fables, being a creative lot, have found various unconventional gateways in and out of the Homelands. Below is a short list of a few of those found:

• Witching Well--Flycatcher and a group of supposedly deceased Fables crossed the barren land of the well to find and found the Kingdom of Haven
• Tree in the business office of Fabletown--Bufkin recently found that the tree links to the Kingdom of Ev, a neighbor of Oz
• Magic Beanstalks linking to the Cloud Kingdom
• Inside of a Dragon in the land of Skold--Boy Blue took back channels when he was in the Homelands. One gateway he did not discover until after he had slain a dragon.
• The Great Train--a magical train that comes through in the "thin places" between the Mundane world and the magical world. It is the way in and out of Americana.
• Jack Frost--after giving back his mother's (the Snow Queen) wintry powers, Jack developed the natural ability to be a living gateway. Once he transports himself to a new world, he needs to let his energy stores replenish before he leaps again.