In the mountains, past Terracina are some very scenic drives: you are high up and have fantastic views of the sea! Eventually you will get to Fondi, which lies on the ancient Via Appia.
The most important sight in Fondi is its castle. The Castle has a round tower standing more than 30 metres high and is the symbol of the city. The castle was built in the 14th century by Onorato I Caetani over a stretch of ancient Roman walls. In the 16th century it was the seat of Giulia Gonzaga's court of literates and artists. Since 1987 it houses the city's museum. However, the opening times are very sporadic. Every time I've been there, it's been closed, even when I've checked on the website! Still, it's lovely to walk around and to gaze at from a local bar and soak up the atmosphere.
Another sight is the adjacent Palazzo del Principe ("Prince's Palace"), constructed in 1466–77. This Is attributed to the Catalan architect Matteo Forcimanya. Its portals, the mullioned window, the court and the loggiato are built in the Catalan-Gothic and Angevine architectural styles.
Head down the ancient cobbled streets to a small square where there is the Cathedral of St. Peter (Duomo, 14th century). This is built over an ancient Roman building identified as a temple of Jupiter. It houses the sepulchre of Cristoforo Caetani, a marble bishop cathedra and a pulpit from the 13th century, an Annunciation Tryptych by Cristoforo Scacco with St. Peter and St. Paul by Antoniazzo Romano.
Itri is a real mix. Modern and historic lie side by side. There is a castle that overlooks the town and can be visited. The custodian was very helpful and informative when I visited. She also loved practicing her English!