There are reasons to be concerned as a single woman (or single person) walking the streets late at night of any city in Spain. Chances are excellent that nothing will happen but one should always exercise some caution, yes.
The last time I was in Barcelona I too was alone and going to late dinners and returning even later. I felt a bit uncomfortable returning so I stuck to the main roads and avoided the narrower pedestrian streets which were more dimly lighted. The place to have the most concert would be the "El Raval" and "Barrio Gótico" districts - of course these are MUST-SEE neighborhoods during the day as they have the most character/flavor.
You'll want to excercise the same caution in Sevilla in the Barrio Santa Cruz or/and any other area where there are narrow, dimly lighted pedestrian streets.
Also goes for large crowds where there are high concentrations of people or where there are tourist attractions where people are distracted and not paying attention to their bags, wallets, or cameras. This situation is ripe for snatch-and-run theives.
You'll probably feel overly cautious your first day or two but after that you'll realize your fear were unjustified. This is usually the way it happens.
Enjoy your time in Spain!
Saludos, MadridMan/BarcelonaMan
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