Piazza Castello was designed in 1564 by Vitozzi. It's lined with porticoes and is the real heart of the city: here and hereabouts stand the great buildings that have witnessed the development of the city, from the ancient Roman era to our days. In the middle of the square stands Palazzo Madama and on the side you could see The Royal Theatre, The Royal Palace, the Royal Library (that houses Leonardo da Vinci's operas). 
In Piazza Castello (The Castle Square) begin the most important central streets, like Rome Street, Po Street, Garibaldi Street and Pietro Micca Street.