Can you illusion a window out of a solid wall?

Question

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.

Answer 1

Yes, because other illusions can do similar things

I am not sure if there is a printed spell that will do this, but I would say it is possible. My reasoning is that other spells can do similar things. What you want to do here is to allow vision through an object. Disguise self allows you to appear a foot shorter, so it must allow vision through your head if you do.

Answer 2

Without illusions: passwall

This spell can create a window, plain and simple, although not a very decorative one. It also has a duration and thus can be dispelled (unlike stone shape). You might have to provide a way for the NPC creating it to make it permanent until dispelled, as the duration is only 1 hour. You could place it in a glyph of warding, but then it will only activate once.

With illusion: invisibility or major image, sort of

Sadly, invisibility can only target creatures, not objects, so you cannot just cast it on a boulder. However, you could cast it on a creature that can imitate one, like a galeb duhr (MM 139). Then, you can get a second surprise, as the now-wall attacks the party. You will need to somehow make the invisibility permanent or give the creature an item that can cast it, and have them use it when somebody approaches. One option is a ring of invisibility, but that might be a bit strong for low level parties. A lower level/rarity item is the ring of spell storing, but you will need a caster to replenish it.

Major image has the advantage of having a permanent duration when cast with a 6th level slot, but it cannot make the wall truly transparent, it can only be made to show what is on the other side. I would recommend creating the illusion of a wall with a narrow window in it* (overlaying the real wall altogether), so that is has depth. Thus the "outside" bit can only be viewed from a certain angle, and the image not changing to match the angle is harder to notice. If the creator is an illusionist wizard and they are around, they can even change the image according to the time of day. If you want the castle to be a standalone creation, you will need to give them a way to automate this.

*You might need two castings here, one for the wall, and one for the "photo" in the "window", as the spell creates the image of only one object.