One of my favorite style dresses from Game of Thrones is the drapey, robe a la francaise/piemontaise type dresses popular in King's Landing--worn by women in town, handmaidens, Shae, and more. It's the perfect dress for the last day of Costume College (or any other convention) when you want to wear something, but be a little more relaxed!
Construction information for this dress, including a diagram, can be found here. The dress is very simple though--it's just a tube of washed silk taffeta from Renaissance Fabrics with drawstrings at the center back and center front, fastened at the center back with ribbons, and worn with a belt. Some construction can be seen on Instagram.