Haustor

Composite image
out of 3 exposures
sky, moon and buildings
Windows lit from inside using flash with different color gels

Thank you all for your comments and favs
with huge thanks to ^MC-Frizzle for featuring my second DD

I'll try to answer to as many questions you asked here as I can:

Making of "Haustor"
pronounced: /ˈhaʊs.tor/
in Serbian and Croatian language its a slang for building entrance

I was helping my friend [link] to make his stunning image Escape from the city [link]

Once we where done. I tried to make an urban stratrail image on the same location by stacking multiple exposures.

unfortunately bad atmospheric conditions combined with light polluted sky delivered sad result with only single star bright enough to create nice trail in the sky.
Result you can see here: [link]

Its stack out of 4 (10min)exposures during one windows where lit from from inside with multiple flashes form Canon 58EXII flash using a different color gel for each store.
During second exposure outside of the building was slightly lit with LED from my mobile phone(bluish shadows of terrace)
Third was the same but without any intervention.
and finally for the fourth I opened the aperture a bit to let more ambient light in.

In post processing after I exhausted all possibilities of photoshop to extract more starlight from the original image. I decided to try making composite.

I took a piece of sky from my image "Space truckers" [link]
5DmkII/15mm fisheye/30sec/f2,8/iso4000
Masked the sky from original image and blended them together.

Result was still lacking that Wow punch(to justify making a composite in my eyes) so next step was adding moon from one of many simple moon exposures that I had in my Lightroom library.

After that only thing left was adding several transparent/white circular gradients to simulate moon shine on the sky blending it all together and finishing PP.

P.S. Taken in Belgrade city center, one cold November night