Herculaneum was the first part of a quinella of ancient sites for us on the one day. My wife and I visited Herculaneum in the morning and then backed-up with Pompeii in the afternoon. Reason being that there is probably a little more shade at Herculaneum and it is a little smaller and therefore more comfortable for the morning jaunt.
Parking is near impossible at Herculaneum. We parked at Pompeii and jumped on the Circumvesuviana train to Ercolano and took the short 15 minute walk down the street to Herculaneum. The guy who was "minding" our car in the private car park along the entrance road to Ristorante Lucullus at Pompeii thought that we would be back for lunch but didn't seem phased when we turned up at 5:00pm - so the strategy was successful!
Herculaneum and Pompeii suffered equally in the 67AD eruption of Vesuvius but in different ways, so it is valuable to visit both sites to do the compare-and-contrast thing. The lifestyles of those living in both sites was also different and that adds some interest. Herculaneum was more the ritzy resort whilst Pompeii was a working port city.
We used a Walks of Italy guide for the Pompeii tour but did Herculaneum by ourselves. It is obviously better with a guide but our Pompeii guide was able to bring us up to speed with some of the detail of the Herculaneum site when we caught up with her at Pompeii in the afternoon.
Enjoy both sites - they are different!!
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