Hi Shayne,
First can I say that's a nice little diorama, especially for a first effort!!
The trees look good, and the postioning of all the diiferent elements. The 88mm does look very nice. I've only done one diorama myself involving snow, funnily enough my first one too! I'm certainly no expert, but the snow looks ok, what did you use?
You asked about the empty space behind the walls? The wood needs weathering and perhaps a wash of somesorts. It looks too clean. Chas was right about the ladder, and you need to add some rubble from the walls. You could use smaller broken pieces from the original walls if you still have any, adding smaller bits, such as gravel, crushed brick, sand and even cat litter is good to use. Broken pieces of wood from the floor boards would also add an element to it all, and help fill the space. In amongst all that, add personal pieces of equipment from the soldiers, boxes, crates and especially spare ammunition for the gun. You could also add 1 or 2 extra troops, maybe making the hot tea for their fellow comrades, there's alot of decent kits out there depicting soldiers at rest etc..., that might add alittle extra to your scene?
Anyway, hope some of that is of some use to you, look forward to seeing how it all turns out!
Cheers for now,
SIMON