Medieval kings are a perennially popular subject for Halloween costumes. They are also widely used in theatrical productions. It takes more than simply wearing a crown to make someone look like a medieval king; however, and since different kings dressed differently, it can be challenging to put together a good costume. There are some fairly standard aspects of king costumes, though, that you can build on to make yourself look truly regal.
Start your costume with a plain, tight fitting shirt and pair of trousers. Try to avoid anything obviously modern-looking, such as jeans, button-down shirts, prints or stretchy material, as well as neon colours. Avoid black as well, as it was not available as a cloth dye in medieval times. Instead, try to find bright, solid colours. These clothes should be as close fitting as possible without the use of stretchy fabric or visible buttons. Try to use natural fibre clothes if possible, as artificial fibre clothes often hang differently and can impair the quality of your costume.
Wear a loose-fitting sleeveless tunic over your shirt. A basic tunic is easy to make from a single large rectangle of cloth by folding it in half lengthwise, cutting a head hole in the centre of the fold and sewing the side edges together except for arm holes. Alternately, pre-made tunics are available at most costume supply stores and re-enactment events. The tunic should hang to around mid-thigh and be a bright solid colour as with the shirt, though these colours do not need to match. Depending on your preferences, you may want to use a richer and heavier fabric for the tunic than the shirt, though as with the shirt and trousers you should try to use natural fibre. Wear a leather belt over the tunic.
Add a cape over the other clothes. This should be made of the richest fabric available; velvet, in particular, is a good choice to create a classic "kingly" look. Capes are widely available at costume supply stores and should come with a clasp or tie to secure them. The cape can be anywhere from hip-length to floor-length. As with the rest of the clothes, bright colours are best, and, in this case, deep red or purple is ideal.
Wear plain, nondescript leather shoes or boots. It is difficult and expensive to make or buy footwear that looks truly medieval, so try instead to keep attention off of your feet as much as possible. Simple black slip-on shoes are relatively easy to find, and if necessary, you can use paint, marker or shoe polish to obscure any brand logos or other designs. Try to find socks that match the colour of your trousers.
Find a costume crown of your choosing from any costume-supply store. Alternately, you can make a simple crown by rolling paper-board into a tube big enough for your head to fit into, then gluing it in place, cutting out a crown shape and painting it with metallic spray paint.
Accessorise your costume richly. Over-the-top use of gold costume jewellery will help add to the regal appearance of your costume. In addition, you may want to hold a sceptre and hang a sword from your belt; both of these items will also be available at costume stores.