Right now I'm designing a dungeon that is actually a ruined castle, but with illusion magic it appears to be restored to its original shape. One of the key parts I need is a window, which is actually an illusion, on a real, solid wall.
The party only needs to be able to look through this wall - so whether it's a glass window or (hollow?) window does not matter - while in a moment later, the illusion will be dispelled and the solid wall will be revealed.
What method can be used to achieve this kind of illusion: a see-thru window on a solid wall?
I'm aware I can just make the effect up, but I prefer a spell castable by a PC (or magic item) because this will serve as some sort of a memorial of a PC.
This does not strictly need to be an illusion, although I strongly preferred it to be one. I only need that, when the effect is active, it shows what's behind the wall. When the effect is inactive, it shows what's on the wall. Maybe some sort of 'glow in the dark' ink, but this actually shows what's behind the wall.
If the method needs a certain interpretation over a rule or description, as long it does not stretch too much from RAW, I'm fine.