echiromani:

Over the doorway of the Quirinal Palace, Saints Peter and Paul. Now the official residence of the President of Italy, it was constructed in 1583 by Pope Gregory XIII as a summer retreat, and has since housed thirty popes and hosted four conclaves. It was requisitioned after the fall of papal Rome in 1871 for use by King Vittorio Emmanuele.

It is the sixth largest building in the world, in terms of area.