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Missile Silos (Silos for short) are a kind of terrain. They can be used by an Infantry, Bike, or Mech unit to fire a missile. The missile can be targeted at any area on the map (even in Fog Of war) and effects a radius of two squares from the target outward, dealing 3 damage to all units, friendly or hostile. Each silo has only 1 missile and cannot be restocked. They provide 3 stars defensive cover, depleted or not.
Groups of enemy units are usually the most suitable target. Designating an armored target over a group of weaker units may make a larger difference because doing 3HP of damage with regular units to an armored unit is harder than doing that same amount of damage to weaker enemies. Targeting a cluster of enemy properties with units on them will not only do damage but also deplete their War Funds at the start of their turn to repair those units.
Advance Wars: Dual Strike mission 'Crystal Calamity' features nine ASDS (Anti Satelite Defence Silos), they work like regular silos, but instead of being fired on the map they automatically target the Black Onyx, a satellite on the top screen. If an enemy unit reaches one of these silos (when still armed) the mission is lost. When all nine silos are fired the Black Onyx is destroyed and the forcefield surrounding the Black Obelisk dissappears, allowing it to be destroyed as well.