Zinaida K.
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Time to visit Piero della Francesca frescos formally limited to 30 mins, but, at least out of season, nobody checks if you leave on time.
The rest of the church worth attention as well.
You have to pay for ticket reservation no matter if you buy online or onsite. In October it was no problem to have a ticket for same day, only couple of time slots were fully sold out.
A backpacks have to be left in a locker at the entrance. There is a toilet inside.