The centre of the old town of Santiago is Plaza de las Armas. A square surrounded by historical buildings, at one side you will see the Cathedral where the archbishop resides. At one side of the square is the post office in a white neoclassic historical building. At the other side is the Museum of National History.
Couple of blocks away from the square you can visit the Museum of Pre-Colombian Art, which is a must (detailed story). Again if you walk couple of blocks from the square, this time at the opposite side, you will reach to the Presidential Palace which is La Moneda. It is where the military coup started with bombs on 11th of September 1973 and President Allende was killed on that day inside the Palace. On the right of the Palace is the Ministry of Justice with the memorial sculpture of Allende in front of it.
Following La Moneda, again another couple of blocks ahead is the Municipalidad, which is the Theatre and the main Opera House of the city, which is used for more than a hundred year and still the main venue for ballet and opera- check the programme for concerts and shows. Both the Theatre and the building opposite are built in French neo-classical style. If you climb another couple of blocks you will reach to the Cathedral ‘Basilica La Merced’, known as a pilgrimage site, and the archeology museum with brown bricks next to it where you will see a collection from the period of indigenous cultures at pre-colonial era.
If you continue towards Parque Forestal you will reach to the Fine Arts Museum. You can also take a guided walking tours that start at the Plaza de las Armas at 10.00 and 15.00 everyday and last about three hours.