You don't have to reserve but I recommend it because people were waiting hours for a tour that didn't reserve.
SURE
You can always walk in. Even on weekends you will find a spot. It's just you have to wait for an hour may be
You can tour with an existing crowd the day of, but I would recommend reserving if you have a limited time frame. I went in the summer and didnt get a slot until an hour later just because it does tend to get busy.
The first time we missed our reserved time and arrived at 4, they told me the next available time slot is sometime around 7. It should depend on date, just for reference.
I doubt that you do but just call to make sure
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