Castello in Sardinian is called Castedd'e susu, which means 'upper castle', because this is what it was and, from an architectural point of view, still is.
Castello is perched on the top of a white rock which looks directly onto the gulf of Cagliari. The site was used in the remote past, though probably not urbanised: the Punic and Roman cisterns built to collect water and the Roman amphitheatre carved out of one of its slopes are just two examples.
It wasn't until the 13th century, when the Pisans began creating their commercial outposts in Sardinia, that the strategic importance of Castello became apparent. They decided to take control of the hill and soon blocked access to it, fortifying the rock with heavy walls and three large towers, two of which are still in pristine condition and are among the main attactions Cagliari has to offer. They also built the Cathedral.
When the Aragonese and then the Spanish took over power in Sardinia (14th-18th century), they kept Cagliari as their capital. They improved and enlarged the walls of the castle, modernising the defensive system. Sardinians were effectively cut out from power and important positions, and were only allowed to have humble jobs during daytime. When the day drew to an end, a horn was the signal for the local population to leave the castle or else be even thrown down from the walls.
The Savoy family took power at the beginning of the 18th century and also made many improvements, but the most radical changes occurred in the 19th century, when Cagliari, and therefore Castello, stopped being a military fortress. Instead of mighty defensive bastions, the new promenades and terraces (e.g. Saint Remy Rampart and Buon Cammino) became the places where the cagliaritani could mingle and socialise.
Although the richer families have left, and the population has decreased over the years, Castello is living a new phase, buildings are being restored, there are new bars and restaurants; art galleries and shops are being opened and it is overall coming back to life, after generations of romantic but decadent neglect.