Can you explain what mercantile governments like Tyrosh, Lys, and Myr are? Are the rulers just like lords only based on wealth and not land.

Well, Tyrosh, Lys, and Myr all have Conclaves of Magisters – i.e, a council of wealthy merchants elected by other wealthy merchants – Tyrosh also has an Archon elected annually from among the Conclave to be the executive.