A medieval banquet is a party that fully immerses your guests into another world. Whether you choose to approach the medieval theme seriously or playfully, it should encompass all aspects of your banquet, from food to decorations to what the guests wear.
Invitations with a medieval theme or tone are available to download online, or they can be purchased. Consider addressing your guests as "Lords and Ladies" in the invitation and also on the envelope. A formal wax seal on the envelope adds a nice touch.
Your decorations will depend a great deal on the tone you are hoping to set with your party. If you want a serious, realistic tone, decorate your room in rich, jewel-toned fabrics and set the table with crockery and pewter tableware. If you are aiming at a more playful tone, buy faux castle backdrops at party stores. Consider hanging coats-of-arms around the dining room, perhaps reflecting the names and ancestries of your guests. Light the room with candles and even torches in wall sconces, and make sure you have plenty of flowers.
Encourage your guests to wear medieval costumes, which they can rent or borrow from friends who frequent Renaissance festivals. Have crowns and tiaras available for guests who don't come in costume, or provide flower wreaths for uncostumed women to wear.
If your banquet will be a serious reflection of the medieval era, research the recipes and food presentation of the times. You will want to feature soups, rustic breads, roast meats (even a whole roast pig if you can manage it), with crystallised fruits, sugared nuts, oatcakes and gingerbread as desserts. Pour ale or mulled wine for your guests, and remind them that in medieval times, most water was unsafe to drink.
Mix together a playlist of medieval music to set the tone of the evening. If you have the room, bring in an expert to teach some medieval dances, perhaps during a break between banquet courses. If your party is more toward the rowdy side and you know your guests will be drinking a fair amount of ale, consider a game of Medieval Musical Chairs, which becomes even more fun when guests are trying to navigate in their costumes. Consider employing a medieval jester, a juggler, or a group of madrigal singers to entertain.