This was a great museum/ attraction. You watch a (very bad) short film, which doesn't really give you faith in the rest of the visit, but that is all put to rights when you get the tour. A volunteer miller will take you up the stairs and explain at each level what was done there, what the mill did, will give you small snippets of history of mills, how mills worked, until finally you are at the very top and on the gallery looking out over Haarlem. Everyone loved it!
My Gran has bad knees and did find the stairs a little hard going. There are a lot of narrow staircases, no lifts, and the tour will take you to the very top. Just something to bare in mind if you have someone a little unsteady on their feet in the group.