You can walk from the Castel Sant'Angelo to the Piazza San Pietro in a straight line along Via della Conciliazione. I think you can't walk over that cool bridge thing that seems to link the two places, but I wish I could have.
As you reach what you'd expect to be an open space, you realize that Saint Peter's Square is everything but that. It's technically open in that there is no roof and there are no real walls, but there are movable partitions everywhere that you have to weave your way around. I guess the square can get pretty busy when the pope shows his face up on the balcony to tell his sheep things and the barriers help keep the place orderly and the people safe. Calm down, I used the word sheep in the biblical sense, I wasn't offending anyone.
Between and around those tripping-traps, however, there's a loooot to see and discover:
And then... then you get in line.