Are there any. . . Vents?

AMOGUS

This is a medieval castle. There are no vents. There isn't even an air system. He's stuck in here.

. . .

Crap I cant make among us jokes sadness

Ok, jokes aside, besides the front door, he cant leave? There arent any windows or holes or anything?

Well, on closer look of the room, he might be able to find a way. It looks like somebody took the most scenic bit of nature in existence and plopped it into the castle. The room is large and domed, covered in trees, grass, and sweet-smelling flowers. A single throne sits at the center. It's coated in moss, but the moss has been cut away to make a pleasant green swirl design. Trees make up the ceiling, and little rays of light poke through the breaks in the leafy canopy. Large windows allow plenty of light to pour through.

There's animals everywhere. Rabbits, like him. Horned rabbits of some kind as well. Deer, and perytons as well. It's a curious mix of human realm and fey realm wildlife, each one having a counterpart from the opposite world. The stranger part is that there's also fey. Many of them. None of them are natives, at least as far as he can tell, but not a single one is trying to leave.

Can he eat his way out?

He doesn't have the bite force to gnaw through several thick trees.

is there any less thick foliage he can chomp through?

He can't climb the trees. He's not going to get out by brute-forcing this place.

Is there any way for him to feasibly leave? At all?

Of course there is. But he isn't going to break his way out. He's a rabbit.