Plaça Sant Jaume is the heart of Barcelona administration, right in the same square fagin to each other we can find Catalonia's government and on the other side Barcelona city hall. Funny enough, in the same square is where the main roman streets of the old Barcino -the name of the city in Roman times-, the Cardo and the Decumanus meet. Nowadays the Cardo will belong to Llibreteria and el Call streets and the Decumanus belongs to Bisbe, Ciutat and Regomir streets.
Right in this place where the two streets crossed is where it was the Roman Forum -totally destroyed- and the Temple of Augustus, that still remain 4 columns from the temple, located at the time in the Tàber hill at Paradis street.
Plaça Sant Jaume is a bit of our protest place in the city, when there are demonstrations -always pacific- or people want to protest about something it is the perfect place as you have the town hall and the government in the same place so they can't ignore it.
Sant Jaume square is not only for demonstrations but it is a big part of traditions and festivities in Barcelona too, there are many castellers -human towers- being done in there when there is a special event such as the city's festivity: La Mercè.
Visit Catalonia Government: Generalitat
If you want to visit la Generalitat on the inside you can but you need to book in advance, there is free entrance and it is opened the second and fourth weekend of every month except in August. All visits must be booked in advance and they are free and guided, the duration is around one hour and you can request the language you want your visit in. I would recommend you to visit it if you have enough time on the city, there is the famous orange courtyard that is beautiful. You can book in the Generalitat webpage visit section.
During 23rd April (Sant Jordi), 24th September (La Mercè) and 11th September (diada) is opened to the public from 10 to 20h as they are public holiday days.
Visit Barcelona's city hall: Ajuntament
If you want to visit la Generalitat on the inside you can on Sundays, there is free entrance and the opening times are from 10 to 13:30h. There are guided visits: Catalan language at 11h, 11:30 and 12:30h, in Spanish at 10:30 and at 12h and for English at 10h. For individual visits you will be given a leaflet at the entrance.
During 12 February, 23rd April and 4th June it is opened to the public from 10 to 20h as they are public holiday days.
Disclaimer: Sant Jaume square picture by David Palleja