Not as qualified to make any remarks on the environment art, but from our playtest and my experience going through the map, the only changes I could suggest while respecting the layout is to make the routes into B a bit more conspicious.
i.e. A vent almost blended in with the wall. Maybe a light source or a border?:[Blocked Image: http://i.imgur.com/uYYYry7.jpg]
The actual engagements and chokes that defend B routes are fine IMO, but it was a bit hard to intuitively know B was beneath the hidden stairwell:
[Blocked Image: http://i.imgur.com/wzrfGpY.jpg]
These things are probably forgiveable since it's just a matter of players learning the layout but it'll definitely make it a bit less confusing.
Good luck!