Herculaneum is on a much smaller scale than Pompeii and can be done in a couple of hours (take water and wear flat shoes). The structure of the buildings survived the eruption of Vesuvius better than in Pompeii, so you can still see the barrel vaulted ceilings of the baths, for example, and the balcony floor of the Casa a Graticcio. There are also some impressive mosaic floors but fewer intact frescoes and artefacts than at Pompeii. A lot of restoration work was going on while we were there, so some of the buildings, including the suburban baths, were closed. If you have to choose between Pompeii and Herculaneum, you should definitely choose Pompeii but Herculaneum is also worthwhile and interesting if you can do both.
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