Mage Bridge

A shimmering, translucent, pink-purple-blue floor of force spanning the gap between the mainland and the largest of the Mage Islands, the aptly-named Mage Bridge serves as the primary connection allowing for trade between Mages and the rest of the City-States Region. 

The Bridge

Constantly shifting through tones of color, giving off a faint chiming sound that intensifies with hollow echoes as people walk across it, the Mage Bridge hangs high in the sky three hundred feat above the swirling waters of the strait below. Crossing about a mile in the air, the bridge feels completely solid to the touch or step. People with any sort of acrophobia are not advised to cross this bridge - it's translucent, and even with the shifting colors is is easy to look down through the barrier and see the waters far below. 
The bridge is essentially permanent: extremely powerful Mages of the evocation and conjuration schools created what is essentially a permanent Wall of Forcespell. In cases of emergency, however, it is possible for the wall to be "turned off". A tower on the island side of the bridge contains a series of gemstones that, pressed in the correct sequence, can cause the bridge to disappear. The mages actually turn the bridge off once a year during the High Convocation, a time during which all non-Mages are refused entry to the islands. They do take care to ensure the bridge is empty, unlike in an emergency.
As a Wall of Force, the bridge is generally invulnerable, though a Disintegrate spell can punch a hole in the wall. Sufficiently powerful magic could potentially shatter the bridge but the magics required to do so don't exist outside of several powerful Mages working together (or perhaps some of the more ancient Elves). 

Trade

As the primary passage between the City-States Region and the Mage Islands, the bridge sees regular passage by carts carrying food and other supplies into the islands. The supplies (and the gold coming back over) generally pass through Magear on the way in; the city prides itself as being the "Gateway to the Mage Islands" and hosts a number of offices that specialize in helping others contract the services of Mages. 
The bridge is not the only entrance to the islands, as a port on the southern tip of the main island exists, but this port primarily is used for quarried stone and other supplies from the southern City-States. It also is used by Mages wishing to travel to the south. Otherwise, the bridge is the primary passage for people and products passing between the two realms.