After the invasion of New York II that killed Eddie Smith, New York II activated a shield barrier to prevent any more attacks by the shadowghosts or Imperials. The shields were active until the Imperial Guard commander, Grievous, led an assault that destroyed the shield. Another version of the shield barrier was used over Metropolitan II.
The shield is the first line of the defense that protects New York II from attack. The shield is impenetrable & takes on a bubble form. Certain areas of New York II could only be entered through mine rails that ran under the ground. The shield could withstand multiple bombardments & wouldn't destabilize unless if a very powerful weapon was used against it. When Grievous attacked the shield with missiles, the explosions made by the shield made it look like he was launching fireworks at it.
In any event that the shield could be destroyed somehow, the city's alarms would set off a second line of defense that would immediately retaliate to the shield's destruction. This was demonstrated when the Imperial Guard destroyed the shield, unleashing a swarm of Imperial target drones that began wiping out the Imperials in seconds after the shield collapsed.
If the shield was hit with weaponry that was eco-based, the shield would set off it's own "third" line of defense by dispersing a shock wave of intense energy that would vaporize any attempting invaders within a 10 mile radius of the city shield.
When Grievous hit the shield with a weapon of mass destruction, the resulting explosion caused the shield to break. The shield first turned a blinding white, causing all of the city to be full of light. Then, as it deteriorated, burning hot pieces of the forcefield fell to Earth as watery wisps of vaporous glass that could burn a person's skin away on touch & decay the cement of buildings in seconds.