The Roman Emperor Domitian built the villa on the shores of Lago di Paola in the first century bc. Lago di Paola is the lake which lies between Sabaudia and the sea, about a 15 minute drive away from my parent's house. You can now visit the ruins but only with a guide. It is well worth it, especially if you enjoy history as I do. However, the tours are only in Italian and only on certain dates and at certain times. Even if you speak no italian, you can go along and look at the ruins, well preserved roman toilets and an amazing underground water cistern. Bear in mind though that there is a lot of walking on uneven ground. Wear comfortable shoes and take water in the summer months. Also, you will need to take your car as all visitors drive to the entrance of the archaeological site in a convoy.
To organise the visit, you must go to the Parco Nazionale di Circeo visitor centre in Via Carlo Alberto, 107, near Sabaudia. This is near the outskirts of Sabaudia near the forest. Enquire at the main desk and they will be able to tell you the dates and times of the tours as well as the cost. Also ask if there is a tour in English, this is unlikely, but you never know!
This under ground water cistern is truly amazing. It's like entering a huge cathedral. The Romans took their water systems very seriously, so the cistern is very well constructed with beautiful arches. It also is lovely and cool, great to get away from the intense heat for a while during the summer!