The Madness castle's unavoidable protections strip Carl's invisibility the moment he reaches the hidden rear door.
Carl knows before he starts that The Madness Castle is surrounded by a few protections he cannot avoid, and expects the effect at any moment. As he closes on the painted-over rear door the system fires: he has been deshrouded, his invisibility negated by the castle's protections.
Carl's reaction is a flat "Oh well." The Invisibility Potion had been for the approach across the trap field, not the entry, and he goes straight to disarming the door's three traps. The deshrouding does not trigger an effective alarm either; a klaxon sounds when he forces the door open and is cut off almost immediately.
Castle defense mechanic that ends the crawlers' stealth approach at the wall.