How was ancient Sparta ruled? There seems to have been a dual kingship, yet Sparta is often considered to have been an oligarchy rather than a monarchy. Can someone please clarify?

Did the way Sparta was ruled remain essentially the same, or did it change? For example, was it ruled the same way when they repelled the Persians as during the EB era?

Also, from what I know, there were to social groups, or castes - Spartiates and Helots. Did the Helots have any role in war? Did they only farm, or did they trade as well?