This map is shaping up to be a very solid first release, but I did notice some issues that break the believability of the space that wouldn't be too hard to fix up. First: very shortly after I loaded the map up I noticed the constant ambient screaming and moaning (slowed down burning zombie?) that really doesn't fit the theme at all, it's more annoying than anything else.
Leaving the ambient sound like that would be like

for this map, IMHO.
There are also invisible walls in a few places that make the map feel constricted; invisible walls should generally be avoided like the plague, if the path needs to be blocked add a visual cue that it's blocked.
This smoke looks very unnatural, in reality smoke doesn't accumulate into a ball in open air, heat makes it rise and it gets carried off by the wind.
This ceiling doesn't look like it would support much, it would need heavier support beams running opposite the floor boards.
The debris in this area sometimes blocks the player and has to be jumped onto.
This invisible wall gives no indication that it's impassable, it actually looks like you should be able to jump right over. With the smashed building next to it maybe you could use some debris to show that this way is blocked.
You can stand on the very edge of this ledge, and with a high-value item there I suspect I'm not the only person who would try to jump onto it. I'd suggest cutting this ledge back to make it unreachable, rather than just correcting the invisible wall.
Last one: I really like this vista, the scale of skybox is very good, and it makes everything blend together and really look like a larger space, but it would be much more visually appealing if there were some more buildings of varying height, right now it's basically a horizontal line across the screen. It also might not be a bad idea to add a few structures further back to fill in the cracks where you can see sky between buildings. The courtyard below could also use some work, as right now it seems like 'the rather empty dead-end courtyard' would be a good name for the area.
Overall the map is looking very good. I think the layout works as it is, and if you add too much more play area it'll just start feeling like a maze. That's just my opinion though.
Also, a belated welcome to the Snarkpit :wink: