Can anyone tell me what time period this dress is from?

Ignore the unicorn.

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Just an uneducated guess.
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It looks like it WANTS to be (at least Hollywood) Camelot-ish.
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I don't know that it's possible to ignore the unicorn. If I had to guess about the dress, I'd say possibly 14th century.

:shrug:

After William the Conqueror invaded he set up Norman kings. It's believed the time set is during the reign of King Richard I (the Lionheart) who ruled England from 1189-1199. At the time Normans were the ruling class and Saxons were the peasants. It was also around the time of the third Crusades which were around 1189 to 1192.

In some of the movies and stories they depict Robin Hood as waiting for Richard the Lionhearted to return from the Crusades and fighting the Sheriff of Notingham who was a traitor to the king. The Sheriff supposedly took all the money from the Saxons for himself and Robin Hood just took it back from the Normans and gave it back to the Saxons.

Another case of taxation without representation.

In the latest movie with Russell Crowe this is the actor who is playing Richard the Lionhearted (Danny Huston)

Oh and the unicorn is more middle earth than middle ages