Its very nice for your first submittion ^^ but if you add more shade on the character it'll looks much better , anyway i admire your hard work for your first time, and i saw this one in minitokyo as well and i do like it.well done.

You did a really nice job on the scan, vectoring it and extending the vines.
I love the background you did, however I think you need shading on the grey bricks. Paint some nice shades into them, for a more creepy look. I also think you should change the grey bricks to a nice dark brown color, I think it would fit more with Yuuki's color scheme.

As for the back, where the gates are thats a nice job on them and a nice touch to making that atmosphere. The back where you have that brown gradient could use some darkening on the top of it.
That would add alot to make it more dungeon/castle like, shading is very
important when you make a piece like this. :3

A few colors leaks on her here and there, not too noticable but remember to always check your vectors throughly before using them in a wall ^^
Great job on this one, hope you continue to make more walls~

The colors are lovely, :) but I think the background is too blurred even if youre trying to create some distance effect. The bg would also of been nicer if you added more details to it. Even a sky through the window would have been good. Also I think the dark brown streaks in her hair are too blurred as well. Blurring your strokes isn't really necessary. Like on her chest bones and lips for example, they look too washed out atm and would have been better without the blurring. Nice job with her line art though.