This article was started to list words and terms that are common today, but were not in use during the early 13th century. For those troupes who want to give a more authentic feel, it is best to get into a mindset that does not use them, or at least to have a reason why anyone in the story would use them. As always, if it's more fun to keep the anachronism, do so.
Assassin The term is unlikely to be in use in Europe yet, though the cult that lead to the development of the word has existed since the late 11th century. In the late 13th century, Marco Polo wrote of the cult, but it's unknown to this contributor if he used the Italian word for assassin or not. Earliest recognition of the term in English at dictionary.com is the 16th, having migrated from French or Italian. An approximation of the word might be used by crusaders in the know. There are likely to be alternatives in some languages, but otherwise, hired killer/murderer, killer for hire, etc should do.
Brute in it's English meaning may or may not have been in use so early. Earliest year dictionary.com gives is in the 1300s, but it's use is known in Middle English, which date's as far back as the 11th century.
Meter as a unit of distance does not exist. It DOES exist as a measure in poetry and music, as in rhyme and meter. It originates from Greek (it is not Latin and not in use in Latin speech), but is noted as being introduced into Middle or Old English, so is likely in use, though mostly among those with knowledge of classical music theory, or at least of some music ability. A note for those of us in the USA, the spelling across the pond nowadays is metre, but is pronounced the same, as in theater v. theatre. Though not a unit of distance, it is possible that magi of a tradition that incorporates or those who speak Greek, might use this in place of magnitude when speaking of magical power.
Thug Developed in the English language as a result of English occupation of India, derived from Thugee. No basis for the use of the word in Mythic Europe.
Zombie Unlikely. More research would be needed to tell how far the origin of the word goes back in Africa, but it is unlikely that European magi would have knowledge of it.
Tomatoes do not exist anywhere in Europe, Africa, or "The East". They may, if your Saga sends magi that far, be imported from fairly far south in South America. So in addition to the food that won't develop until after Marco Polo brings some ideas over from China, there are no tomato based products of any kind.
Potatoes do not exist anywhere in Europe, Africa, or "The East". They may, if your Saga sends magi that far, be imported from the New World. They will not be introduced into Europe until the 1500s.
Christmas trees Pulling again from the Wikipedia article, Christmas trees weren't in use for at least another two centuries after 1220. Although there are now legends attributing the practice of using Christmas trees to St. Boniface, these legends didn't crop up until after the Christmas trees, and are thus not myths of 1220.