This site, unlike the more popular Pompeii, Herculaneum and Oplontis where you will trip over brown "ancient monument this way"  signs the second you get off the train, is very hard to find. First be assured, it does exist, (even though anyone you approach will deny all knowledge of it). See the Soprintendenza Archeologica di Pompeii's excellent website.  http://www.pompeiisites.org

It involves a 25 minute walk up a very steep hill. The postal address for GPS purposes is Passeggiata Archeologica,  80053 Castellammare di Stabia NA, Italy. Directions as follows:

 Exit C di S Circumvesuviana station. You will be facing the sea and promenade. Pass to the right of a  newspaper  kiosk in front of the station concourse.Turn right into Via Regina Margerita (300m). Right into Viale Europa. This is a steep main road. Cross it carefully. This is one road where the drivers seem to ignore the pedestrian crossings. Just before the top of the hill do a sharp left into Strada Varana (not Via Varana like it says on the mpa) a quiet narrow and rather rubbish strewn country road for about 400 m.  Merge onto  Passeggiata Archeologica. Various pizza places etc are on your left.  You will see finally sign posted on the left down  a nameless country lane "Villa Arriana". Apparently Villa St Marco is down the next equally nameless country lane on the left just before you get to the Grand Hotel La Medusa. .

 Villa Arriana is stupendous and amazingly intact, with beautiful frescoes and what must have a been a commanding position on a cliff just above the sea wall, but when you get there you will immediately realise why the lack of signage. It is a small (in site terms, the villa is huge) only recently excavated site, only lightly protected by roped off areas and you can only go round under the supervision of the custodian (who is super garrulous by the way and has opinions about anything from local cheese to whether you can be a doctor without studying latin - you might try to get him to stick to the subject if you want to get round the entire building inside of 2 hours ). If they made it too easy hoardes would turn up and it would be overwhelmed. The only facilities are the custodian/guide, toilets, and some people who turn out ot be taxi drivers hoping for business on the return journey (which is down hill all the way). Do persevere  and pat yourself on the back when you arrive hot frustrated and slightly intimidated.