Source: Warhammer Fantasy: 6th Edition

Movement (Appendix Four - Rule of Siege)
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Models defending a castle are organised into units just as they would be in a normal Warhammer battle. In theory, a unit in a siege battle can be as large or as small as you like, but you will find that between five and twelve models is the most convenient size.

All troops (including the besiegers) can adopt a skirmish formation when deployed on the ramparts or towers to aid with movement. The rules for skirmishing are repeated here for your convenience. Some exceptions to the normal skirmish rules will apply, as noted below.

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