What does a painter want?
He wants an empty wall, big as a church, that everyone will see, and freedom to fill it any way he chooses. The Sistine Chapel had two big windows in the wall above the altar but the Pope agreed right away to having them bricked up so Michelangelo wouldn’t have to paint around them and incorporate them in his design; he would have a nice, big, flat, empty wall to paint. And it would be right above the altar, where everyone faced, where they had to look. And the pictures would stand right-side up—not like the ceiling scenes, which made you spin around all the time to study them, and crane your neck. For once he was going to have things his way.
The Sistine Chapel (Wikicommons photo published here under the terms of the GNU General Public License as published by the Free Software Foundation)
What was that—what was his way? He would make hundreds of male nudes in every posture and perspective he could think of. He would skip all the props: no more marble pedestals or moldings or any architectural motifs at all. No more drapery. Nothing but Promethean heroes, triumphing or falling. He wouldn’t worry about a background either: there would be none. Behind the multitude would be air, just empty space. He would paint it blue and the blue all over would unify the fresco. Lower down the wall, where Hell was, the blue would turn into a cheerless dim light.
Last Judgment (a public domain photo published here)
The subject didn’t matter much. Originally it was going to be a Resurrection. He would have done a Resurrection. He would also have done an Ascension or a Crucifixion or an Assumption. At his age, these things didn’t matter. His style was fixed, he knew what he wanted. He knew it isn’t the “what” you paint so much as the “how”.
But it just so happened that this was a subject after his heart. He’d been reading Dante for years, ever since he was a boy at Lorenzo de Medici’s palace; and he knew The Divine Comedy by heart. Knowing your Dante was knowing all about the demons and the horror of Hades or Hell. The tumult, the excitement, the terror, of the Last Day was just right for a collection of the kind of swirling, rising, cringing, creatures Michelangelo liked to imagine.