No it is not
No just turn up
We have just been. We had no problems just turning up (on a Wednesday afternoon in early May). I can imagine it would make sense to book at popular times.
If you also want to visit Maeshowe Tomb - you definitely have to book in advance.
Historic Scotland offer a combined ticket for several of the archaeological sites, which could save you money.
I think you could wing it and be fine.
Just turn up
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