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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 "high value" units like 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, such as their Ruler or a caster directing a Tower's Air defense spells. Heavy units, including Mounts, can screen for their riders on their own initiative.
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.
The size of the screening unit(s) and their number also seems to play a factor in establishing a succesful defense. Wanda and Jack avoided a hail of deadly arrows during the Siege of Spacerock by standing under a newly Decrypted Red dwagon. On another occasion, Parson calculated that he didn't have enough casters screening for him (3 at the time) to successfully run accross a clearing full of Thinkamancers (7 at the time).
Shields may aid when actively screening, both to intercept attacks and for the screening unit to take less damage and screen longer.
Larger units are better at screening.
The number of screening units doesn't just block blasts or arrows spatially, but adds some formula modifier to the evasion or defense of the screened unit.
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.
- ^ Blocking projectiles - IPTSF Text 20, Fritz screens Delphie against Frenemy fire.
- ^ Air Defense - ibid, Screening Delphie was done by Chief Warlord Fritz, and not some piker.
- ^ Mounts vs Magic Items - LIAB Text 47, "...a magic item would not fight or take an arrow or a blast for you, the way a proper mount would."
- ^ Trail Blazing - LIAB Text 52
- ^ Fliers screening - LIAB Text 47 (relevance?)
- ^ Versus Airspace - WB2014 Digdoug - Episode 15
- ^ Doombats - TBFGK 113
- ^ Dwagon umbrella - LIAB 51
- ^ Number to screen - LIAB 71