It is generally safe to walk around, both day and night, especially in the city centre and surrounding neighbourhoods. Even the dark and narrow streets of the historical quarters don't represent a serious threat. Although crime does exist in town, it is not gang-related and does not normally consist of mugging, assaults or random beatings. Having said all this, it is also worth pointing out that some precautions should be taken. It is common sense not to venture out too far away from the centre at night, on foot and in dodgy parts of town. If you see a group of people you don't like the look of, maybe it's a good idea to just cross the street. Cagliari is not the sort of place where you can just jump on a passing-by taxi. If the situation gets particularly edgy, call 112 (Carabinieri) or 113 (Police) for help. If possible, find someone who speaks Italian to help you make the call since there is no guarantee of finding an English speaker on the other end.