The old part of the city (called Castello, the castle) lies on top of a hill, with a wonderful view of the Gulf of Cagliari (also known as Angels Gulf). Most of its city walls are intact, and feature the two 13th century white lime-stone towers, St. Pancras tower and the Elephant tower. The local white lime-stone was also used to build the walls of the city and many builidings. D.H. Lawrence, in his lively memoir of a voyage to Sardinia, Sea and Sardinia, undertaken in January 1921, described the effect of the warm Mediterranean sun-light on the white lime-stone city and compared Cagliari to a "white Jerusalem".

The Cathedral was restored in the 1930s turning the former Baroque façade into a Medieval Pisan style façade, more akin to the original appearance of the church. The bell tower is original. The interior has a nave and two aisles, with a pulpit (1159-1162) sculpted for the Cathedral of Pisa but later donated to Cagliari. The crypt houses the remains of martyrs found in the Basilica of San Saturno (see below). Near the Cathedral is the palace of the Provincial Government (which used to be the island's governor's palace before 1900). In Castello is also the Sardinian Archaeological Museum, the biggest and most important regarding the prehistoric Nuragic civilisation of Sardinia. Finally, Castello hosts many craftsmen workshops in its tightened and scenic lanes.

The Basilica of San Saturnino is one of the most important Palaeo-Christian monuments in Sardinia. Dedicated to the martyr killed under Diocletian's reign, it was built in the 5th century. Of the original building the central part remain and the dome, to which two armes (one with a nave and two aisles) was added. A Palaeo-Christian crypt is also under the church of San Lucifero (1660). This has a Baroque façade with ancient columns and sculpted parts, some of which found in the nearby necropolis.

The Sanctuary of Our Lady of Bonaria was built by the Aragonese in 1324-1329 during the siege to the Castle in which the Pisan had took shelter. It has a small Gothic portal in the façade and in the interior houses a wooden statue of the Madonna, which was thrown off by a Spanish ship and landed at the feet of the Bonaria hill. The cloister of the convent is home to the Marinery Museum.

The other early districts of the town (Marina, Stampace, Villanova) retain much of their original appeal and still seem to function as distinct villages within the town.

A testimony of the Roman domination is the Roman Amphitheatre, carved into a block of rock (the typical lime-stone on which Cagliari is built). The Amphitheatre still stages open-air operas and concerts during the summer.

The districts built in the 1930s spot some nice examples of Art Deco architecture and some controversial examples of Fascist neoclassicism, such as the Justice Court (Palazzo di Giustizia) in the Republic Square. The Justice Court is close to the biggest town park, Monte Urpinu, with its pine trees and artificial lakes. The park includes a vast area of a hill.

Cagliari has one of the longest beaches in an Italian town. The Poetto beach stretches for 13 km. and was famous for its white fine-grained sand.

Cagliari has one of the largest fish markets in all of Italy with a vast array of fish for sale to both the public and trade: the Mercato di San Benedetto (S. Benedetto Market)