We walked in and paid at the door
Because of times entries, it's better to buy online, but you can take the chance and buy at the entrance. Unless they are already at their max, you should be able to get in.
Purchase when you arrive
Yes, you can purchase tickets at the door upon your arrival.
Both
You can buy a ticket when you arrive, but they recommend buying one online in advance for a specific time.
You can buy a ticket at the door, unless they have some scheduled event
I'm not sure I was there for a wedding
When we visited recently, we purchased a timed ticket in advance. The people in front of us, though, just walked in and purchased one right then. So... you can do it either way. The weekdays are not busy.
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