Location for the party

The location is always important in a theme party because if you find the right venue then half the work with setting the mood is done. I have disvocered that in Denmark it's far easier to find rooms from around 1700 than medieval times and I have seen some medieval couples settle for these locations. But the fact is that gold mirrors and traces of Roman/Greek architecture have nothing to do with the middle ages and it's a shame to have a lot of beautifully medieval dressed guests when they'll look awkward in the all too modern halls.
There are a variety of choices though fitting for the theme, some more expensive than others but all equally good.

The Castle:This is probably the most expensive solution but very convincing for your guests. Many old fortresses or castles will lend their rooms out for weddings, even those which are presently museums. The downside is that you rarely are allowed to bring with you your own drinks and food, so the owners of the castle will have you pick a menu from some fine restaurant they are working with. Usually this means very fine dining but not medieval meals. The castles also often allow you to sleep there with the entire wedding party for an extra payment. You really can't get closer to the knight like scenery than a fortress but this is mainly possible in a European context, I am not so sure about America. Just remember that not all castles are from medieval times, and some was renovated later.
Here is an example from Denmark. It's a fortress in Kolding, where you can rent the old halls.

Outside, in a big tent:The cheaper but also very wonderful option demands some attention to details that you will have to deal with yourself. You have to rent a tent, additional toilets and a kitchen tent and a generator for power (if you aren't near any of those facilities.) Depending on the time of year you may also need some heaters. The floor will also often be rented by itself and then of course you need table, chairs, personal, plates and so on. There is a lot to juggle but it is also a wonderful way to throw a medieval party. You are close to nature and can set it all up in any location you might choose (if only you remember to ask permission if the land is not yours). So you can dance the night away in beautiful landscapes/forests/fields. This may also fit a knight but peasant will also do nicely.

On a re-enactment medieval fair:All over the world people who are interested in medieval come together each year in markets where all dress according to the fashion of the time and sell items related to medieval. Often a wedding is welcomed, but you will probably have to deal with a lot of attention. It is possible to eat away from the crowd as the other participants usually have an area where the public won't enter. It is the ultimate way of setting the right move and if you aren't too shy you can get married in the middle of it all. Obviously it makes your wedding less private, but all locations have their disadvantages. It will probably not be very expensive and you can prepare a whole roast suckling pig on a grill (very easy and medieval-ish).

In a barn:Often old Manors have these huge barns that are a perfect setting. There can be difficulties finding one with proper kitchen facilities, but it is worth is. They have an air of the ancient halls of knights and with some torches and flower wreaths they can look just right. Sometimes the ones you rent it from will already have chairs and tables and maybe even a chef with at wedding menu. This will also fit both the rich and the less rich theme.

Medieval manors:There are also some old manors from the medieval who will rent out their rooms for weddings. This might not be as fancy as the castle, but definably also a choice to be considered. Sometimes they have their own restaurant and as with the castle they often have accommodations for sleeping guests. These manors often have these feast halls that make a great background for the reception or party.

Restaurants who specialize in medieval food:They are rare but they do exist. In Denmark we have one, which is a part of the Medieval Centre Museum on Lolland-Falster: the Golden Swan. I know of at least one other in Budapest called Sir Lancelot Medieval restaurant. This arrangement is the one that we chose as it had
1: the medieval environment, 2: the exact food they ate in medieval times 3: the experience with this sort of theme 4: The management has a lot of contacts in the re-enactment business and might help you with other details, such as clothing, 5: All is included as opposed to the tent solution where you have to get the facilities, the location, the food and much more by yourself.
We are also getting married on the sight as it is beautifully located by the ocean. The example is the Sir Lancelot medieval restaurant.

Museums:The last recommendation is medieval museums who likes to rebuild some houses like they appeared back then. Sometimes you can rent their biggest replica house, (if it's big enough to fit everyone). It won't always be written on their homepage if they rent out, so try calling or visiting to ask. Sometimes there will be a cafe or a restaurant who can provide the food, and on other occasions you will have to get a catering firm.