How did the reserve concept develop? You mentioned in some secret histories that a mechanic like that already existed in (the) dark ages, but were there differences between the old and the new variant, or did you completely take over the old variant?
Royal Carriage in Dark Ages (called Procession in its day) was essentially identical to Royal Carriage as published. It didn't refer to a mat. Okay I dug up an image, and in text form it's:
Procession: Action, $5
+1 Action
Set this card aside.
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While this is set aside, when you play an Action card, you may put this into play. If you do, play that Action card again after it resolves.
I don't remember the particular spark for that card in Dark Ages. Probably it was "what if there was a Throne Room you could save until you needed it." We liked the card but it needed a mat and couldn't have one.
For Adventures, I was looking at what a Seaside sequel could do. Seaside had mats, though it didn't get much use out of them. One obvious nice thing to do with a mat was to save cards for later, like that Dark Ages card. And why wouldn't that work; it had already worked on that Dark Ages card. So I tried some out.