In the Valley of the Tirso river, in Fordongianus, is the old 'Forum Trajan'. In Roman times it was the most important town in Sardinia's inland, surrounded by a fortification system designed to contain the raids of the Barbaricini (a population coming from the mountains of Sardinia). Ruins of the remains the thermal complex, with a swimming pool that still collects the hot water from the hot springs, are visible together with remnants of porches and foundations of various buildings.

Fordongianus, the river Tirso and the Roman city are mentioned for the first time in the work of the geographer Ptolemy (Ist century B.C.); it was referred to in its most ancient name, Aquae Ypsitanae, due to the presence of natural springs of hot water, which still flow at a temperature of 56 degrees Celsius. The water had with great therapeutic properties. For this the city was from its beginning an important SPA location, but being located on the border with just Romanized territories it was also an important military garrison.

In mid-imperial age under Emperor Trajan (98 - 117 AD), Aquae Ypsitanae took the name of Forum Traiani and became a great trade center for goods, as it was strategically located on the main road axis of the time, that linked the cities of Karalis Turris Lybissonis and Tharros.

To this period belong the remains of Roman baths built between the 1st and 3d century. The thermal complex consists of group of buildings focusing on the main pool, the "natatio", used for therapeutic purposes. Nearby was also an artificially heated steam bath.

Not far from town is the very interesting and beautiful, in its simplicity, Church of St. Lussorio. It was built by the Vittorian monks in the twelfth century. Of great interest is the crypt beneath the church, which, with the deposition of the Martyr Lussorio, a place of worship for the whole island.

In Fordongianus itself , is the so-called "Aragonese House", built in the mid 16th century, a rare example of the mansions of the time. Located at the center of the village, it is characterized by bright red of the local trachyte stone and decorative elements of Gothic-Aragonese style.