Not bad!
Little things to help you out...
1) You need to move your origin to a more central spot so you can draw off the vertexes.
2) The far right wall has a line in it, if you erase that line and the whole wall disappears, that means the wall edges are not parallel with each other.
3) Something in general, when you do your initial alignment you are supped to run the red and green handles along the walls, It helps align, just make sure green and red are perpendicular.
1) moved the origin to the bridge/door now to make it more central (didn't realize it'd move my whole model haha never mind)
2) so
thats what the problem was!! I've tried re-doing the lines but I cant get it to fill in the surface. Is there a way to make the line draw parallel?
3) when I did the alignment I ran the green lines along the bridge and the red lines across the back wall. Looked ok to me but as you're mentioning it I'm guessing I must have got it a bit off?
One more thing - how much detail do I need to go into here? Obviously more is better, but take the walkway side rails for example. I can cut the holes out so it looks more like a fence rather than a flat wall, but do i leave that flat or expand it to make it more of a box (like ScottMcTony has done)?
Thanks for the feedback guys