If you happen to have a formal/high-quality medieval/fantasy-themed outfit (think Lord of the Rings, Game of Thrones, or the White Queen), then you're very encouraged to wear it! Just no out-of-period costumes like pirates or Victorian dresses, please. Otherwise, long dresses with classic silhouettes would be a great fit (see the example dress in the banner photo). Feel free to have fun accessorizing them with themed items, too, like cloaks, medieval girdle belts, and necklaces! The bride and bridesmaids will be wearing pale sage/mint green and white, so please avoid those colors along with any ultra-modern neon colors or animal prints. As to footwear, just remember that the reception will be outside, so be sure come in shoes you can wear comfortably on grass!
If you happen to have a formal/high-quality medieval/fantasy-themed outfit (think Lord of the Rings, Game of Thrones, or the White Queen), then you're very encouraged to wear it---even armor! Just please no out-of-period costumes like pirates or Roman legionaries. Otherwise, vests and slacks would be great. Feel free to accessorize with themed items like cloaks, sword belts, or medieval-style cufflinks/collar chains.
Costumes aren't required, but if you've been looking for an excuse to buy a good one, then we really recommend ArmStreet (a Ukraine-based costumer) for high-quality options. We're also fans of DressArtMystery, who made my wedding gown, and FolkoftheWood on Etsy. These websites can be kind of an investment, though, so feel free to take ideas you find there and look on other sites for more options! We just ask that you look for decent-quality materials and mature/formal costumes---we're mainly hoping to avoid Spirit Halloween-style vibes, haha.