These are the notes regarding allies on the GB Swordpoint page:
Allies act in every way as a normal part of the army.
When fielding Allies, only the troops native to the Allied list can be used - not the allies of the allies!
You may take Allied Commanders (other than an Army General) but may not exceed the total
number of Commanders available to the host list.
Where a minimum is shown for a unit type in the Allied list i.e. "+2 Horse Archers", then these units must be taken as Allies before any other unit.
Any restriction, limitations or options in the Allied list i.e. 0-1 Guards, the unit must not be more than four bases strong, every second unit may be fielded as skirmishers etc, apply.
All troops imported from the Allied army count only against the Allies percentage and are not affected by the host army’s proportions. For example, the Viking list has no Cavalry component but it can take Irish or Saxon cavalry from its allies.
However, you may not take more of a troop type from the allied list than it allows to its own forces. For example, a list allowed 50% allies cannot take 50% allied cavalry if the allied list only allows 25% cavalry.
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