[Andusciassus]

I'm with Magus_Arts on the mud-thing.
It seems like you need to spend some time with reference-pics to make the mud more beliviable. It looks quite a lot like something from someones first piece of scenery...the rest however does not.
It's a great looking piece you've got going there. The floor is fantastic and the gravel at the blasted part makes it close to perfect.
A few words of advice though.
First it's really important to hide the fact you are using plain cardboard. On the shots from above, where the top of the walls are showing, it becomes mighty obvious and I assure you, no matter how good your model will ever be that cardboard edge will always be a certain doom. It'll ruin your ruin so to say.
Get some glue and small gravel to the top of the walls, on the horisontal little edge, not all over. If you have any woodfiller or such it's even better.

Second advice is about the windows.
They look just like cut out holes at the moment. If you add some 3D effects around them like frames and tills. It'll make a huge differance. It's also a great way to fix any uneven edges and to hide the cardboardy-look I mentioned earlier.

Third is on the mud.
If your model has been sealed with a good base colour or PVA just go ahead and use many different colours. Vary the Drybrushing and Washes techniques and don't be afraid to make mistakes. You can always hide them with more paint. Go gentle in each go however and use a piece of cloth, a sponge or even your fingers to spread the (thin) paint.
THen make sure you give the mud a direction or movement. Who dragged it in there and how did they move?
I can imagine they went mostly from the door to the blown out corner. Make sure this movement is readable in your model.

Next, fifth, comes detailing. Add some minor hightech bits and pieces in places where they appear to serve som purpouse. A few consoles and a light. Some wires and such. It'll add to the feeling of scale and general 40K-iness.

Lastly is the small scenario-like traces. An empty helmet and a gun standing next to a window. Maybe an empty waterbottle. Something that gives the observer a picture of this model being a part of something bigger, it'll take it from a piece of scenery to becoming a piece of narrative, a setting.

Sorry for the somewhat lengthy post, but I have not been posting in a while and got a bit carried away I guess. It would never have happend if your model didn't have the potential it has however. It's a very promising piece of scenery and for being a first-time thing, it's simply amazing.

Good luck and keep up the good work.