Natural Monument since 1993, the Acquafredda hill and its medieval castle dominate the Cixerri Valley, in a site important for flora, fauna and landscape.
Tradition attributes the construction of the suggestive castle of Acquafredda to the counts of Gherardesca, in the person of the infamous Ugolino, remembered in Dante's Inferno. In fact, in the absence of reliable documentary evidence, the structure of the fortification can be hypothetically placed in the first half of the 13th century.
The Donoratico della Gherardesca came into possession of the S/W part of Sardinia after the fall of the kingdom of Cagliari. With the imprisonment of Ugolino in the Tower of Fame, the castle passed into the hands of the Pisan administration and later, in 1326, it came under the control of the Crown of Aragon.
A document that establishes the demobilization of the troops still staying in the castle dates back to 1408, now useless on an island almost entirely conquered by Aragonese troops. Having become a fief, it was entrusted to Pietro Otger and was later established as a barony in the 16th century. It is not clear if the castle was then habitable. The first documentary evidence of the state of abandonment dates back to 1785, when the castle was defined as “destroyed”.
The fortification is divided into three levels: the village, the tank tower and the actual castle. The village was the lowest part of the whole complex and included a series of rooms suitable for housing servants, troops, stables and warehouses. The whole thing was defended by a crenellated wall, about 80 meters long, in which the traces of four towers and the patrol road have been identified. A tank was also found in the village, turned into a barrel and made of brick bricks.
On the second level, a tank tower is clearly visible, also turned into a barrel and divided into three rooms that could be completely filled with water.
At 256 m high, the actual castle is located, today in an advanced state of decay despite repeated restorations. It had to have a 'U' floor plan and three floors, one of which was underground and two were elevated. Nothing remains of the main tower while the watchtower, located a little lower than the entrance, is preserved.

History of studies
Given the importance of the castle of Acquafredda, numerous studies have been conducted since the end of the 19th century: see the entry “Siliqua” (1850) by Vittorio Angius; a few years later it was Carlo Brundo's contribution. Mario Pintor's study and Foiso Fois's article are more recent. The latter also contributed to the volume “Castles of Medieval Sardinia” (1992). The article by Donatella Salvi was published in 1995, while the one by Stefano Castello and Simonetta was published in 2000.

Bibliography
 V. Angius, ''Siliqua'', in G. Casalis Dizionario geografico storico-statistico-commerciale degli Stati di S.M. il Re di Sardegna, XX, Torino, G. Maspero, 1850, p. 153;
E. Lucchi, ''Il castello di Quirra e la torre di Saralà: visioni di Sardegna'', in Mediterranea: rivista mensile di cultura e problemi isolani, 6, 1932, n. 1, pp. 32-36;
F. Fois, ''Il castello di Quirra, rocca dei Carroz: contributo alla storia delle fortificazioni in Sardegna'', in Studi Sardi, XXIII, 1973-74, parte I, pp. 217-227;
C. Brundo, Il castello dell'Acquafredda. Scene storiche del secolo 13, Cagliari, 1878;
M. Pintor, Acquafredda: il castello del Conte Ugolino, Cagliari, 1962;
F. Fois, ''La storia fra i ruderi: il castello di Acquafredda: un ammasso di rovine'', in Almanacco di Cagliari, 1978, senza pagine;
F. Fois, Castelli della Sardegna medioevale, a cura di B. Fois, Cinisello Balsamo, Amilcare Pizzi, 1992, pp. 49-58;
R. Coroneo, Architettura romanica dalla metà del Mille al primo '300, collana ''Storia dell'arte in Sardegna'', Nuoro, Ilisso, 1993, sch. 173;
D. Salvi, ''Siliqua, il castello di Acquafredda'', in Archeologia medioevale: cultura materiale, insediamenti, territorio, 22, 1995, p. 393;
S. Castello-S Sitzia, ''Il maniero rivisitato: nuove ipotesi sull'origine del castello di Acquafredda presso Siliqua'', in Almanacco di Cagliari, 2000, senza pagine;
G. Serreli, Tutti i castelli dei quattro regni, "Darwin. Quaderni", n. 1 (luglio-agosto 2006), pp. 104-109.

How to get there
To reach the castle of Acquafredda, take the SS 130 and travel 35 km. At Siliqua, take the SS 293. After 4 km you arrive at the foot of the Acquafredda hill, which you must climb on foot to reach the ruins of the castle.