Too plain.
No really, it would be nice if we would have had something to look at.
This drawing is made out of only 27 lines, and only five different colours, so there aren't any details to look at. Things like the text on the door that is just a line or two makes it unfinished.
Spend more time on detail, but most of all, check your art one last time before you submit it. Out of the already very few lines, most of them are a few millimetres too long, meaning they stick out.
By the way, I'm not an art expert, but it seems to me that the line under the grey parts to the sides means that it's all one wall, without an opening. It seems to me though that you wanted to have an opening there in the wall.
Talking about the openings, something should come out there. Maybe just a hand, but at least something, to give it for example a bit of a scary feeling.
In general, it almost seems that I spent more time on this review that you on that art piece.
It's 3D, sure, and for practicing it's a well done picture. But it isn't a great picture to just look at as art.