From that one screenshot I see a few problems. You're creating geometry without nodraw/dev textures.

The skybox is not in proper areas except against the back wall.

Then again its a small alley that doesnt show much.

I made an example alley for something to do and to help out a bit. I am no where near a pro mapper. but just a few ideas you could consider when mapping.
Also never hurts to do a google image search for ideas:D
a useful habit i do is i make everything using nodraw texture and then just texture things after using the single face texture tool. this makes is so there aren't a lot of unnecessary textures.

I also only made a box for the skybox as it isn't going to be made into a real map or anything so it was just easier.

Here is the over head SDK view.
that yellow line is a invisible wall that keeps people from going out of the alley, it gives the look that there is more than just this alley and then nothing which makes it look more realistic when looking around. ( yes the wall is the wrong way i fixed it after i took the screenshot)
sdktop.png

Here are a few screenshots of it in game. I recommend you add some trash/debris around as it is a alley and there would normally be a mess around. otherwise it could be a bit empty looking.

in some alleys the windows aren't really close to the ground so you could move them up a little, I indented them as it would add some look to the alley so it wasn't just flat walls, tapered in so it wasn't a drop to the window and it fit better. also added a roof so the walls just don't end at the top.


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Also if you want people to rate your work you should post some in game pictures, more screenshots, or even a close up picture. makes it easier to see