Continuity mistake: As shown in the Batttle of Kandor flashback, when Brainiac's ship begins forming the shield around the city, the shield begins forming from both above the city at the ship, and at the ground around the city going up at the same time and the two haves meeting in the middle. Then when Superman is trying to break into the ship when he found it on an alien planet, Brainiac's ship begins the same process of the shield. One shot even shows the bottom half forming and moving up under Superman. But just a shot or two later from in front of Superman, you can see the ground of the city at the edge and there is no shield there.
Picture added by Quantom X