There is a lot of back story there, and you get it in the comics. The brief is that a powerful enemy that they call "The Adversary" started to take over the Fables' homelands. They fled in front of advancing armies of bad guys wanting to "subdue" (but really kill them). This "Mundy" world has no interest to the Adversary, so the Fables are safe here.
Any more info and we get into spoiler territory. The comics have amazing, richly detailed and excellent storytelling. They come VERY highly recommended.
As ElToppo said, they were chased out of their own worlds by a growing empire back in the middle-ages by mundy calendar. Most of the Fables in Fabletown were smuggled to our remote, obscure world by either Bigby or Bluebeard, which is the main reason Bigby was afforded a chance to redeem himself and Bluebeard is obscenely rich, and the refugees gathered to some extent and founded Fabletown in the middle of what was then New Amsterdam in the early 1600s. The worlds are connected by linked portals here and there, and all the known portals connecting the mundy world to any of the Homelands are closed and heavily guarded on both sides by Fables.
The reason no attempts have been made to invade our world is that it's pretty much barren of magic, making it seem harmless and useless to the technologically stunted, high fantasy-esque lands the empire already occupies. There's an annual celebration in Fabletown to commemorate their survival and the vague hope of someday reclaiming their lost homes, but most are resigned to life in hiding among the mundy population. And yes, the majority are north of six hundred years old, and no, excepting children growing up, and even that excepting Pinocchio, much to his chargrin, they don't seem to age at all, which is weird given that their physical ages otherwise vary realistically.
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Any more info and we get into spoiler territory. The comics have amazing, richly detailed and excellent storytelling. They come VERY highly recommended.
The reason no attempts have been made to invade our world is that it's pretty much barren of magic, making it seem harmless and useless to the technologically stunted, high fantasy-esque lands the empire already occupies. There's an annual celebration in Fabletown to commemorate their survival and the vague hope of someday reclaiming their lost homes, but most are resigned to life in hiding among the mundy population. And yes, the majority are north of six hundred years old, and no, excepting children growing up, and even that excepting Pinocchio, much to his chargrin, they don't seem to age at all, which is weird given that their physical ages otherwise vary realistically.