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A defensive Screen is a basic battle tactic where units place themselves between an enemy and a friend. Usually "low value" units like Infantry screen to protect Warlords from enemy infantry, though they may also attempt to block archery and ranged magical attacks. Warlords and Casters can also screen for others, though considering their value to their side it's likely they do so only for more valuable targets. Like their Ruler, or a caster directing a Tower's Air defense spells. Heavy units, including Mounts, can be very effective at screening thanks to their large size.
Screening is useful for more than just defense. For example, it can be used to allow an Archer or Caster to shoot at valuable enemy targets uninterrupted. Screening for a warlord can also clear a path for them to reach a valuable enemy target by engaging enemy units that might tie up that warlord. 
Flying units are also capable of screening, and may be required to properly defend a ground unit against threats in the Airspace. Doombats for example are used as cheap screening by Transylvito warlords.
Shields may aid when actively screening, both to intercept attacks and for the screening unit to take less damage and screen longer.
Only stacks that are being led can choose to screen, otherwise they behave like unled stacks. So an incapacitated warlord who hadn't given orders that they be defended or screened if knocked out would result in their stack reverting to behaving unled even though they would rather defend him.