the city looks nice, but withs such a bright sky, it just doesn't happen in reality where building tops are completely pitch black. humans eyes will always adjust, so I suggest you raise the ambience to a blueish 5 or 10. there IS moonlight.
also- good luck with 15 hours, TECHNICALLY I can build a map that takes 1 hr. to complete BUT A- that map will have trouble building.
B- it will have to sacrifice details. so 15 1 hour maps will be too long, dry, and boring for a casual player. you have to understand that players like differing in level design and media. nobody wants to walk through streets killing the same zombies with the same weapons for 15 hours. you need to build a balance which keeps the player being rewarded for each task they perform (increasing in difficulty) and this can't increase the game's file size. I would suggest shortening the game to around 4 hours maximum, 2-2.5 is more like it (there's no way to stretch out gameplay in a level and run it on an average comp without LOTS of keys and aimless walking) I would also suggest keeping enemies constant and different. the people who are playing your game will understand the trade off between number of different enemies and slight lack of detail in outdoor design.
bottom line-
it'll never build unless you lower your expectations, and if it does build, it'll be too long and dry for anyone to complete.
keep it short, keep it fun, and kep the player coming back for more.
(also, fix the shadow from the street lamps, an object emitting light does not cast a shadow of it'self)