Den Gyldne Svane - part of a museum and very serious about their food. Presently they are cooking like it's 1401 AD. It's a restaurant for everyone in the day time and only open for parties on request at night.
Sweden:
Sjätte Tunnan - The year is 1435 in Stockholm, open in the evening and they have a cool website. They do host parties I believe. The rooms are very cosy and sets the perfect mood. The menu also looks very good, all from goat cheese baked turnips to hog with rosemary.
Spain:
La Casa Grande - My spanish isn's sufficient to tell you what's on the menu, but I do know that they offer a medieval experience in a big banquet hall fit for queens and kings. If you can't afford the castle this is difinately an option. From what I can gather, it's a part of a museum.
Anno Domini 1471 - in Praque is not the huge royal hall but rather the small intimate medieval tavern, that in spite their modest appearence holds the possibility for an amasing wedding for the food range from royal to peasent. Like many of these specialised restaurants they only use ingrediences avaliable in the 15th century and base their menu on recipies of the time.
Italy:
Gallo Nero - I don't know any italien, but their website caught my eye.They have a medieval menu for a medieval couple and nice rustic surroundings with dark wooden tables and candlelight.
Belgium:
Restaurant Cosy - is situated near the castle Chateau Bouillon and also offers rooms for the night. They also have a very fine describtion of how a medieval banquet would unfold, the seriation of the courses and what medieval source they use for their recipes.
Estonia:
Olde Hansa - in Tallinn is a beautiful restaurant with a wide range of choices for your wedding banquet, such as Royal Hunting Feast or Merchant Guild's Saturday Feast. They also have a small shop and live medieval music from time to time.
Canda
L'Auberge du Dragon Rouge - yet another medieval restaurant with the facilities to hoist a wonderful medieval party. They offer 5 different banquets, but you better check out their website yourself :) my french isn't the best either.
Latvia
Rozengrâls - lies in the city of Riga and on their menu they have small explanations of where they got the recipe from. It's my impression that it's a big place that could easily acommodate a wedding party.
Please do write to me if you know of any restaurant that offers the same medieval magic in your country so I can add it to the list.