Contrary to popular misconception, Saint Peter's is not a cathedral, since it isn't the seat of a bishop. The Emperor Constantine, the first Christian emperor of Rome, ordered a basilica to be built on Vatican Hill, the site of the present St. Peter's Cathedral. That was completed in 349AD. Construction on the present structure began in 1506, and completed 120 years later in 1626. Saint Peter's was built so that all those who enter would feel the presence of God, (And the power of the church). I'm not a practicing Catholic, but believe me, you feel both!
Just to give you a sense of scale, the four posts at the corners of the main altar are 90 feet tall, and the church can hold 60,000 people! It's dome, largest in the world, was designed by Michelangelo. The picture quality suffers a bit from a lack of light in the basilica. These images were taken with an ISO set to 2500 creating quite a bit of noise in the shadows.
One of Michalangelo's masterworks the Pieta is in the first chapel on the right as you enter the basilica. The sculpture was put behind bulletproof glass after a mentally disturbed man attacked it with a hammer in 1972. The damage was repaired using a portion of the marble from the back of the sculpture.