The dome of the Duomo of Siena. Construction sites during the thirteenth and fourteenth centuries and additions during later ages

The construction of the Duomo of Siena was a long saga spanning several centuries, but the period between the twelfth and fourteenth centuries was without doubt the period with the most architectural experimentation. It began with a large building constructed in limestone, probably covered by a trussed roof and with a crypt which extended from the apse to the transept. Later the construction of the body of the basilica was done with naves and pillars in white and green marble and covered by cross vaults. Lastly, an intermediate cathedral was constructed with the edification of the dome in 1260. The new baptistery offered the opportunity to enlarge the apse and transept and during this work they decided to start the Duomo Nuovo project, which would transform the cathedral under construction until then, into the transept of the newly designed building. This new work was started but not completed, and they therefore returned to the previous work to finish the transept and the apse started earlier.