I was there and found several instances of people using wheelchairs, motorized carts, and even on person on crutches. The ground is either hard packed dirt often topped with hay to keep down dust, or well-trodden grass. It can be a bit uneven, but I never saw anyone having difficulty. Plenty of helpful folks if needed. One woman even decorated her's to look like a little pirate ship!
The Faire is held on a large farm field. Most find that it is able to be navigated in a wheelchair, especially if you have help with pushing.
Almost all of it is, yes. The ground can be very uneven, though, so keep that in mind. It is a large outdoor space that gets quite dusty.
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