This is a quiet day out from Rome and costs 1.50 Euro each way on a Metro ticket. Entry is 8 Euros unless you are a concession - which is 65 not 64 as it says on the site pdf document. This is an interesting site full of almost complete buildings. The site map 2 euros is just a map so you need access to other sources of info to explain the buildings. It is recommended that you take a picnic except there is an absence of seats until you get well into the site, (at the Forum and in the Forum baths). A lot of the site is fenced off due to flooding, not surprising considering the history! After the Vatican and the Colosseum it is delightful not to be surrounded by people in a rush following a flag, or selling trash. (Just a busker on the train).
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