The Sydney Opera House, which was built in 1956 and finished in 1973, is a very interesting example of architecture of the 20th century. It was designed by the Danish architect Jorn Utzon, who won the design competition out of 230 members. His vision was to create more a sculpture than a building. Because of construction and engineering problems he decides to give up the project before it was finished. In this case the opera is meant to be a sailing ship in the harbour of Sidney. According to the lecturer the Sydney Opera House is unique because the professor is not able to classify the building in one of the different categories of architecture of the 20th century. In her opinion it is neither an example of organic architecture nor one of the high-tech revolutions, although it was constructed with help of computer systems and its designer was known for working at organic architecture projects. The opera is known worldwide for its special and new way of look. Till today there do not exist comparable buildings, which underlines the uniqueness of the Sydney Opera House.
which was built in 1956 and finished in 1973,
if something 'was built', this implies the construction was fully completed (in your sentence, in 1956.) Hence, you can't then talk about a much later date of completion.
"The Sydney Opera House, with construction begun in 1956 and completed in 1973,..."
or some less prosaic phrasing than mine!