At the start of the Close Combat phase, before any blows are struck, both players must choose Unit Strength 10 worth of models (with a minimum of 1 model regardless of Unit Strength) from their unit to form the courageous assault party that will fight in the assault (if the unit numbers less than Unit Strength 10, there is no choice). The attacking player will pick their assault party first.

A character that was not one of the models chosen to take part in a building assault cannot be one of the models that step up to replace casualties suffered in the assault.