I'm fairly sure there is no dress code called the "Royal Touch", and it's likely they are just trying to have some fun with their choice of words.
I have heard that phrase used when talking about the Royal Wedding or "Royal Touch Wedding Dresses", so it is possible they are trying to tell you to dress as nice as you would for if you were visiting royalty, going to a wedding, or going to a royal wedding.
Personally, I would just ask them to clarify as I've never heard of a dress code called the "Royal Touch" before (neither has Google), so it would not be unreasonable to ask for clarification.