As other reviewers have explained, non muslims cannot enter mosques in Morocco, so the madrasa offers an almost unique opportunity to see historical Islamic architecture. The building is not only beautiful, but also gives an insight into the lives of the students who studied here. There is a joint ticket for the Madrasa and the Museum of Marrakech, which is less interesting, but worth the few extra Dirham (I wouldn't pay full price for the single entry ticket to the museum only).
If you get a bit lost on your way here, don't accept directions from "Guides" (there are no official guides) as this will result in demands for money, or a trip to a shop, which will overcharge your to pay your "guides" commision. If you want directions ask a shop keeper or a restaurant receptionist (and express interest in returning after you have seen the madrasa to shop or dine).
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