Source: Warhammer Fantasy: 6th Edition

Shooting From an Elevated Position
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A model situated in an elevated position (ie, anything that is more than 2" above the table surface, such as an upper floor of a building) may freely pick any target he can see and shoot at it. The exception to this rule is that if there are enemies in the same building and in line of sight of the shooter, he must shoot at these, as they present a more immediate threat.

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