You can walk in on your own as long as you go in at the hours the graveyard is open for.
You don’t need to book a tour. At the entrance you can get a map of the cemetery but it’s not necessary.
This was a last minute visit for us. They were open and we were allowed to enter. No tickets were needed. But it was a very interesting experience, as most cemeteries are. For me it is very surreal and solemn a yet enriching because of the age!!
There are certain hours it is open (a staff member stays at the entrance and counts visitors as well as remind folks not to touch the fragile headstones and stay on the path) but you do not need to book a tour for entrance.
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