The entrance to the Slytherin common room was recessed inward—though doorways were supposed to be like that. According to Vitruvius's writings, towers protecting city walls should protrude outward.
Ancient murals were all flat. It was not until the Renaissance, when people mastered perspective, that flat paintings gained a three-dimensional effect. Perhaps many years ago, those painting masters had also racked their brains before blank walls like they were now, wondering what to paint on their "canvas."
Before the entrance to the Slytherin Common Room was an iron manhole cover that looked like a sewer grate. It was the very thing that transformed into a gigantic silver serpent.
So far, no one had thought of sneaking into the Slytherin common room, so no one had heard what would happen if its defensive magic were triggered by accident. At any rate, it would not be like the entrance to the Hufflepuff common room, where one would be sprayed all over with vinegar.
So why didn't that quill simply draw a door directly onto the wall? Why go to the trouble of imitating the entrance to the Slytherin common room?
Pomona pounded her head with her fist, as if she could knock some inspiration out of it.
"Don't rush," Severus said from beside her. "We'll figure it out."
Pomona was practically about to cry, though there was nothing much to cry about.
"Don't worry. We have other ways," Severus said.
"Yes, brute force!" Pomona said angrily.
He shut up.
Then Pomona began thinking again. A battering ram struck directly at a city gate. Muggles could not use illusions to hide the gate away.
And if it were merely an illusion that interfered with sight, touching it with a hand would reveal what was real and what was fake.
The wall around the entrance