Defensive energy shields protect all protoss units and structures, and this is, perhaps, their greatest advantage in battle. During the Aeon of Strife, protoss warriors used focused psionic energy to sheathe themselves with impregnable energy shields. Over time, conclave scholars and templar sages learned to reproduce the energy shield by means of induced psi-field generators, which allowed even the smallest robotic machine to surround itself with a protective field. Protoss energy shields are effective at stopping all manner of physical and energy attacks, but they can be depleted through sustained damage. Given enough time, a shield will recharge to full strength.
All protoss units and structures are equipped with plasma shields. When either units or structures come under attack, full damage will first be applied to the shields, if they haven't yet been depleted. Once shield energy is exhausted, the unit or structure will start taking health damage. There's two ways to tell if shields are active:
The top status bar (blue) when selecting a unit or structure will indicate the amount of remaining shields.
- When shields are hit by an attack, you'll notice a light blue energy field shimmering around the unit or structure.
Shields will regenerate relatively quickly when out of combat, but health will not (a primary protoss weakness). Therefore, when micromanaging your units, pull back just before their shields are fully depleted. Shields will recharge and the unit can then return to the fray. Cycling units back and forth can maximize their effectiveness in an engagement.
As with weapons and armor, protoss shields can also be upgraded at the forge. Each level will add more damage resistance to the shields of all units and structures.
Though this upgrade is very powerful, it is also more expensive and does not have as noticeable an effect on the outcome of an engagement (as weapons or armor do). Thus, it is rarely researched first in competitive play – but adding shield upgrades in the endgame will give your units that extra advantage to overcome your opponent.