An energy field is a defensive barrier of energy used to protect whatever is contained within it.
The first known energy shield was projected from the Iron Curtain device developed by the USSR during World War II. It proved to be impenetrable to all weapons of that age, and it is theorized that it may not be able to be penetrable at all (although the field could only be maintained for a short time).
The next major energy field was the Firestorm barrier employed by GDI during the Second Tiberium War. It was indented to protect entire bases instead of individual units or buildings. Like the Iron Curtain, it could only be maintained for a short time.
With the arrival of the Scrin, several more energy shields have been discovered. Unlike the Iron Curtain or Firestorm barrier, the alien shields will collapse after absorbing too much enemy fire. They can be maintained indefinitely unless depleted, though. Heavy Scrin units such as the Annihilator Tripod, the Reaper Tripod, the Devastator Warship, and the Planetary Assault Carrier can be equipped with Forcefield generators. This shield is able to absorb damage and a single EMP blast, effectively doubling the resilience of the unit.
The Scrin sub-faction known as Reaper-17, known for their aggressive tactics, has equipped even their Harvesters with Tripod-derived forcefields. These Shielded Harvesters are the only Scrin unit that naturally comes with forcefields.
However, the Scrin sub-faction known as Traveler-59 do not utilize energy fields, since they value mobility over durability.
GDI's heavily defended Tacitus Containment Building in the Rocky Mountains is protected by an energy barrier.
Reaper-17 and the baseline Scrin faction can research the Forcefield Generators upgrade at the Technology Assembler. Once available; all Annihilator/Reaper Tripods, Planetary Assault Carriers and Devastator Warships gain a spherical shield that protects the host from enemy fire.
The shield integrity is shown by a cyan bar, located beneath the health-bar. However, it will dissipate under prolonged attack and collapse. It can also be instantly shut down by an EMP strike.
If the host retreats to safety after the shield has been depleted, the shield will recharge after 30 seconds.