When the horizon is contained in the composition of your photograph, a lot of attention has to be paid to it. The first thing is to make sure it is straight. Then where it is positioned in the vertical part of the frame is another thing to be mindful of (read about that photo tip here). Add to your horizon checklist the search for intruders at the edges of the frame. In the above photograph, only part of the condos on the left are shown. This makes them intruders to me. Either have the condos completely in the frame, or not at all. Now, on the right of the horizon are some trees. These are part of the natural landscape and I do not view them as intruders. The trees do not breakup the natural flow of the horizon when looking at the photograph, but the condos at the edge do. Buildings are what you need to look out most for at the edges of the horizon. Overall, always pay close attention to the horizon when composing a photograph for the best results.