Montjuïc is, together with the Tibidabo, one of the mountains of the city of Barcelona where you can find an old fort (18th century) at 170 meters above sea level. Until recently it housed a military museum, closing soon after 47 years. Part of the pieces will be transferred to Sant Ferrán Castle in Figueres. The building will be adapted as the headquarters of the International Center for Peace.
To get there you have different options: walking, subway, funicular, bus, car, bike... But I recommend walking. This mountain has different pathways to climb to the castle and go through a very little known part of Barcelona.