Most people dabbling in Medieval cuisine for the first time are not prepared for the difference. Most commercial feasts supply bread, salad, fruit, roast veges and beast on a spit, which is mostly harmless, but far from medieval.
Things to avoid:
beast on a spit. Unless done by experienced professionals (who charge an arm and a leg) they hardly ever come out right. Overcooked, undercooked, dry, late, you name it. It's putting all your eggs in one basket, and can lead to great dissatisfaction with the whole feast, for all that it's visually interesting. Also, the best feasts provide a variety of meats, cooked in a variety of ways.
Tomatoes, potatoes, chocolate, bananas, corn, coconut etc were not around in Medieval times. Think of where things come from before you use it. Today's Middle Eastern cuisine involves a lot of tomatoes, which have to be avoided.
If you want to be a bit more authentic, without scaring everyone, this is what we recommend: Start with Divers morsals: bread with honey and herb butter, nuts, fresh and dried fruit, cheeses, pickles. If your budget stretches, add salami, olives, hummous, tabouleh. Divers morsals is a good way to take the edge off appetites if you have a limited budget.
There are two ways of continuing the feast: in served courses, which commonly follow themes, or in a "groaning board" (smorgasbord) style. How you choose depends on your priorities.
Savoury dishes include: soup (turnips and barley are good to add), stew (spiced with mace for difference), roast meat and veges (chickens are always served whole, pumpkin, carrots, parsnips, onions, perhaps with a honey butter glaze), mushroom pies, herb salads, corned meats, sausages, sausage and saurkraut, whole fish, steamed mussels, mousse, sauces (mustard, strawberry, apple).
I always recommend a roast pig's head with an apple in it's mouth as centre piece. It may not get eaten, but it has great visual impact, and sets the scene.
Sweets include: rose pud, dried fruit compote, stewed fruits (prunes with wine are fabulous), shortbread, gingerbread, custards, tarts.
Medieval cooks loved to show off, and make food art, called subtleties. Some weren't subtle, some weren't edible, but it is a chance for fun, and to help make a scene, especially for a feast served in courses.