Located in La Rovira, in the neighbourhood of Carmel, there is an old air raid civil war bunker built in 1937. The Museum of History has put information boards to explain the history of this place.
You can visit it and enjoy the views of Barcelona. There are not many people that know this spot, so you will not find a lot of people there.
From the bunker you have a 360º sight of Barcelona: the sea, the streets of Barcelona, the city lights… the sight is gorgeus by day and by night, so choose at what time do you want to see beautiful Barcelona!
If you want to get there, the closest metro stop is Alfons X (L4, yellow) and from there you can either walk or take a bus for two stops.