Such a fun theme! We photographed a Medieval wedding a few years ago, the Bride had a purple gown made special for her by a costumer in Wales, who does gowns and costumes for the movie industry. They made the groom a suit of armor! It was amazing. All the guests were also suited out by the same costumer, but they rented theirs. As the photographers, we also went in costume,, it was one of the most fun weddings ever, lol. I love weddings where everyone really gets into the spirit of things :) They had the wedding in Portland Castle, so it really suited the event well. The groom's transport was an off duty firetruck, and the bride had a Beauford I think, the top was down for the drive thru Weymouth and she got alot of attention, lol.
As I remember it, the costumer also rented banners, and other props for the decorations.
Enjoy, it sounds like you have a great day planned :)
Teri M