In the New Age, shields were urban forcefields designed to protect citys from attacks by intruders by the means of invasion or bombardment. Shields formed a translucent bubble that appeared every time something attempted to breach it & any bombs or mortars launched at it can disintegrate them & cause them to explode.
The forcefields were transparent particles meant to stop anything from entering or exiting the area surrounded, though certain checkpoint areas allowed passage through the forcefield. The particles are meant to stop solid macroscopic forms of matter, meaning that other forms of matter, such as liquids, gases & microscopic solids, can pass through it, ex. weather (sand, snow, rain), etc.. The particles are electrifyed & that's what holds them together. The molecular structure is combination of water & silica along with other elements that give the forcefield it's shape & transparency. Because the shield can allow weather to enter the city, the citys in or around the Wastelands cannot use shields as they do not provide protection against the sand storms, yet shields can be used to protect citys by securing the security gap above the citys that have security walls to protect against such hazards. If shields are hit with special types of weapons, the shield might collapse, causing plates of boiling hot water & glass to fall to the bottom of the area. In this state, the pieces would burn people to death & decay cement buildings.
In 2165, after the invasion of New York II, the city placed a shield over it to prevent such an event from reoccurring. However, the theory that the shield was indestructible was proven false as an attack by the League of Imperial Cybernetic Hybrids caused the entire shield to collapse. Later in the year, when Metropolitan City was reconstructed, the city had a shield placed at the security wall spires to protect it from aerial attacks.