Bridging is a spell that allows us to travel from place to place. Inside our castle is a room we call the Zero Room - it can focus a lot of light magic - and we cast a spell that creates a magical bridge. Using the Zero Room allows us to use much less of our spark to cast the spell. And from there, we can bridge to the Realm of Magic below or even back into the natural world (where we do most of our fighting).
Now, when you think of a bridge, you might think of the Golden Gate bridge or London Bridge or some other grand construction. Well, let me tell you, a bridging spell is nothing like that. It creates a portal that we physically move through. Usually, it feels instantaneous, and sometimes it is. But other times, hours can pass before we end up at our destination. No one seems to know why. Perhaps it's because certain places are more connected to the magic. Perhaps it has something to do with how much spark a wizard has available.
All I know is this: It works! It gets us from place to place. But, our bridging spell is only for wizards. We can't try to bridge normals - there's no telling what would happen. But since normals generally don't spend much time fighting those pesky creatures summoned from the abyss, I guess they don't have anything to worry about.