Found this:

"Days of free admission for all visitors
Sundays in the period between 1 November and 31 March
the first Sunday of every month, except for July, August and September (when the first Sunday is holiday, then the second is the free admission day, etc.) "

Think this applies to all archaeological sites

Oh and if you think you're there on the free day and there's somebody in the booth charging don't be surprised. You can on these days walk straight past but they're there to take the money off those that don't know!

Thanks folks .

It used to be free every Sunday a number of years ago but when we went back a couple of years ago on a Sunday in July you had to pay. Think it was about 6-8 euros each but not sure. It didn't appear to be optional but maybe we weren't "in the know". Either way it's worth it for the views.

its really cheap though.. and your money will be spend well. Even on a 'free' day its well worth a donation