"Mercs" as defined by the RTW engine have no ties at all, and can be hired by anyone. Mercenaries in the ancient world were generally different from the modern idea. For instance, while they fought for money most would fight only for a certain group or cause, rather than just anyone.
So most of the "mercenaries" in the ancient world are in game as trainable units, because it better reflects in reality for whom they fought.
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