In this scene, there are skulls painted onto the doors because it helps people to avoid those with the Red Death, a fearsome disease that nobody wants to catch.
Here we see a welder welding shut the abbey gate. He is doing so to make sure nobody goes in or out, keeping the prince and his guests safe.
In the abbey, there are 7 rooms, all with matching color schemes except the black, with its fearsome blood-red colored light streaming from the windows. Nobody wants to be there on account of their fear.
Instead, they all party in the other rooms, although the ebony grandfather clock regularly interrupts them with its disturbingly dissonant ring.
Prince Prospero then notices who seems to be a very distasteful person, dressed up like a corpse that died from the Red Death.
He pursues the person through all the rooms, with everyone too terrified to help, until he suddenly falls dead at the feet of the Red Death.