The Crossroads are an area where all the D/W/U/etc.s meet. It is organized somewhat like an airport. You can just go through one portal, find where you want to go, and go through. You don't have to pay, however.
Certain portals (the means of getting from one D/W/U/etc.) have put down some rules for entering their realm. For example, you would not allowed to carry an ion cannon into Middle-earth. It would tangle the strings of time and space.
Stay tuned for a lovely history of the Crossroads from Akira.
