Medieval king costumes have plenty of uses aside from dressing up for Halloween--many Shakespearean plays include a king, as does the musical "Camelot." There are three basic types of medieval king costumes to choose from: the short tunic and leggings of the early medieval period (King Arthur), the long tunic of the high middle ages worn by crusading kings (King Richard the Lion-Hearted) and the short jacket and trousers of the late medieval king (King Richard III). If you want to include accessories, a crown, sceptre, cape or shield are always appropriate for a king.
Early Medieval Period: King Arthur
Although we have little way of knowing whether King Arthur was a real person and, if so, what he would have worn, we can tap into popular culture's idea of what such a king would have worn. In the Dark Ages and early Middle Ages, royal men wore a short, full tunic and cape over close-fitting tights or leggings. The leggings are easy to find---you can wear dance tights or buy a pair of women's leggings in virtually any department store. To create the tunic, you'll need a pillowcase and a long-sleeved shirt, which will become your undershirt. Cut a slit in the short seam of the pillowcase for your head. Cut two armholes, one in each long seam of the pillowcase. You might want to paint or stencil heraldic symbols on your tunic; a medieval king's crest would have included the colour purple, indicating majesty and sovereignty.
Add two finishing touches with a cape and a crown. The cape is simply a long rectangle of fabric either tied around the neck or sewn to the shoulder seams of your tunic; a solid-colour sheet or tablecloth would work well. As for the crown, measure the circumference of your head with a measuring tape and cut a crown from cardboard. It can be a simple band, or include points or fleur-de-lis flourishes---it's all up to you. Next, spray paint the crown gold or cover it with silver foil. Your footwear should have no visible lacings or heel--ideally, this would be a flat, solid boot or slipper. Put it all together, and you're ready to rally the Knights of the Round Table as their brave king, Arthur.
High Medieval Period: King Richard the Lion-Hearted
This warrior king went on Crusade in 1147 and wore a very particular costume to do so. Medieval Crusaders dressed in chain mail, a sleeveless knee-length white tunic and a cape. On the front of the tunic was a red cross, internationally understood since the time of the First Crusade to be the symbol of a religious pilgrim. Instead of chain mail, you'll wear a black long-sleeved shirt under the tunic, as well as black trousers (jeans are OK; they won't show much, if at all). Tall boots and a crown will finish the outfit.
The tunic is the important part here. You'll need to construct it from white fabric--an inexpensive flat sheet will do. Measure yourself from neck to knee; this is how long your tunic will be. Measure yourself from shoulder to shoulder; this is how wide the garment should be. Mark your dimensions on the sheet in a rectangular shape, and cut two of them. Sew them together at the sides, remembering to leave room for a baggy armhole on each side. It's OK if the tunic looks rough--you're on Crusade, and even the king endured harsh conditions during the long march. Next, add the red Crusader cross to the front of your tunic. You can do this with craft paint or with red felt, cut into the shape of a cross and sewn to the front. Fashion a cape as described above, grab a prop sword from a costume shop and you're ready to go on Crusade as Richard the Lion-Hearted.
Late Medieval Period: King Richard III
This is perhaps the easiest of the three costumes to construct since you can get away with wearing clothing you might already own. Richard III is a great Shakespearean villain, and popular culture usually portrays him dressed all in black as a reflection of his evil, cunning personality. You'll need an all-black outfit, including cape and shoes, topped with a crown. The longer and wilder your hair can look, the better. Royal and noble dress for men in the late medieval period was characterised by lengths of chain draped from shoulder to shoulder, like a necklace. Buyi a length of chain from any hardware store and weari it like a necklace or sew one end to each shoulder.
In this period, tunics had given way to short jackets called jerkins. If you have a plain, simple jacket in black fabric or leather, this will work. Don't use anything with visible zippers, as these hadn't been invented yet; the plainer your jacket, the better. If nothing in your closet works, try a plain black long-sleeved shirt. Your trousers should be plain black cotton (preferably not denim), without embellishment such as fading or whiskering. Pair these basics with black boots and a black cape, and you're ready to wreak havoc as Yorkist usurper Richard III.