Siena's cathedral, the Duomo, begun in the twelfth century, is one of the great examples of Italian romanesque architecture. Its main façade was completed in 1380. It is unusual for a Christian cathedral in that its axis runs north-south.
This is because it was originally intended to be the largest cathedral in existence, with a north-south transept and an east-west aisle, as is usual. After the completion of the transept and the building of the east wall (which still exists and may be climbed by the public via an internal staircase) the money ran out and the rest of the cathedral was abandoned.