You can take 150 buses from Spain Square to Montreal Castle, because the castle is located on Montreal Hill. The roof viewing platform of the castle can enjoy the Mediterranean Sea View and Barcelona City Scenery. Ticket price is 5 Euros, if you are under 27 years old, you can buy discount tickets. The castle is not big. It used to be a place for prisoners. Now it's a museum. The most important thing is to have a view of Barcelona. On the way to Montreal Castle, you can also pass through many scenic spots such as the Milo Foundation, the Olympic Square and so on. If the time is very abundant, it is recommended to take the bus up the hill and then slowly walk down the hill, and browse the scenic spots along the way. Many local people also come here for a picnic. The scenery is quite good. Finally, you can enjoy the night view of the music fountain in front of the National Museum of Catalonia at night. Of course, you should pay attention to the crowd in the square to protect your property.