As you can see in the photographs above the GND wasn't straight but tilted, you can see little lighter areas.I do not really mind this as long as it is not too obvious. It produces a slight halo effect, increasing the contrast between the building and the sky, without blowing out the sky.
However you can make mistakes really easy. The most important thing, according to me, is that it looks true. One thing you should pay attention to is that you do not darken the buildings. With the photo below I messed up. But due to the darker part of the colloseum at the top it isn't really disturbing, even with this flaw i like this photograph. I particulary like the colours, how the blue makes a great contrast with the colours from the buildings. And that the colours from the colloseum kind of come back in the houses in the background.
In the evening i got back to the colloseum. This is something i certainly recommend when you visit a city, cityscapes look very different in artificial lighting. And most of the times there has been done some thinking about how to light important buildings. I found a small square which was a bit higher than the street. The street caught my attention when i was in the daylight at the colloseum because it made a turn just before it.
The traffic was fairly unpredictable because there were traffic lights just behind me. So i closed my aperture down to f/16 to get a shutter speed of 30 seconds. This way it wouldn't show up in the picture if there weren't as many cars as I hoped for. This approach did quite a good job, but if you look closer there's a traffic light in the middle of the picture as well. It turned out that I had to do 6 exposures before i got the picture i was going for.