Solarpunk

Yes, that’s what I said, but the king needs some way to be a middleman between soldiers and cake.

They used actual middlemen yes.

Otherwise the soldiers can just be warlords. Yeah, kings do try on the whole “we are ordained by god” or whatever, but I think the only thing which sticks is the knife.

Well, lords becoming warlords did happen. But more than the "ordained by God thing", aristocracy was a system fueled by loyalty and a value that was more important to aristocrats than money or military might: honor. A kind of reputation. Had you failed your duties, no one would obey you anymore. Yes, some became warlords, and considered traitors but all in all the honor system worked remarkably well and the threat of violence was there but very indirect.

“necessary” would imply that it would exist without the king, which is not the case.

An interesting theory. Did you know that for a long time kings of France tried to impose their currency and their coins but that it failed because an alternate traders currency minted by monks (called livre tournois) was more popular among merchants than the official king's currency? (livre parisii). No, money appeared because people wanted to trade things with people they did not know.

Err no, they do it through money?

Are you prevented to change employer? Is the employer allowed to physically punish you? If no, calling it slavery is an insult to actual slaves. It may be exploitation and still be pretty bad, but slavery exists and means something specific. "wage slavery" is a metaphor.

Also, most sane countries have personal bankruptcy laws and limitations to the amount of debt you can legally contract (with loaners being in the wrong if they exceed it) especially in order to avoid that type of slavery.

Money only has the purpose of exploitation, it is only a proxy for violence.

Well, that's just wrong. Money is a proxy for violence but it is not exclusive. People using local currencies or crypto-currencies for instance are not backed by violence.

If you didn’t have to pay your debts, then how exactly would a monetary system work?

Money is actually about paying upfront, not contracting debts, not having to trust people you don't know. The first currencies were made of precious metal so that the value of the exchange happened on the spot. Paper money is a debt towards the state which has a known track record in paying its debts to the point that people now just attribute value to the paper and debt itself.

But do not confuse "debt by the state" and "debt by individuals". The fact that paper money is technically debt does not make it unreliable, nor does it make it backed by violence. It is actually backed by the belief that it is exchangeable for useful goods and that its value wont vary too randomly.