Popular restaurant and bar in Castello, with amazing views over Cagliari.
While technically not a rooftop bar at all, the street level Libarium Nostrum holds an outdoor terrace which such a view, it is well worth a mention anyway.
With an elevated position in the Castello area of Cagliari, here you get one of the best views in Sardinia, with amzing panoramics over pretty much all of Cagliari.
Both restaurant and bar, Libarium Nostrum serves everything from great cocktails to nice Italian food as both lunch and dinner. The best bet is propably to visit for an aperitif around sunset, which is truly fantastic from here.
Note that you will most likely not be alone, as Libarium Nostrum is a highly popular venue, both among locals and outside visitors.