On the Duomo Square, opposite the old hospital of Santa Maria della Scala, rises the huge majestic Cattedrale dell'Assunta (the Cathedral of Saint Mary of the Assumption), a splendid example of the Italian Gothic.
The building, begun in 1230, replaced a previous cathedral of the IX century, entitled Santa Maria.
The cupola (or Dome) was added in 1264, but in the 1300s the church was
completely transformed: the central nave was raised and illuminated by trifora (three-arched) windows, the façade was worked on by, amongst others, Giovanni Pisano, the chorus was changed, and above all, the transept was widened, in the ambitious attempt to transform the cathedral into the biggest temple of Christianity. Of this last insane widening, begun in the first half of the 1300s and interrupted by the terrible plague
of 1348 (as well as by the collapse of some of the structures), there
remain traces of the structures effectively built on the left side of
the current Duomo: the so-called 'facciatona', the columns of
the three naves and a part of the left side, where you can see what is
certainly the most brilliant door of the Sienese Gothic. The door opens
onto the staircase that leads to the lower Baptistry of San Giovanni.