The horizon line is sometimes known as the eye level as it is always level with the eyes of the person viewing it. That said, if you are standing looking straight ahead on a flat surface at say, the sea (the sea is a good example as it shows a flat horizon) and there is some one of the same height standing ahead of you, their eyes will be level with the horizon. It doesn't matter how close or how far from you they are. Their eyes will still be level with the horizon. In drawing, this is known as 'hanging' figures on the horizon line.
In your drawing, the characters heads are all at the same level below the horizon line. This gives the impression that they are all of the same height an are shorter than the person viewing them (your audience). In general we take the horizon line to be an average human height.
People are all different heights, reflect this in your drawing. It will make it more dynamic and you could use it as a way of leading the eye of the viewer to the most important points of the image.
I hope this has made more sense to you.
Glad to help
Breck
My other tablet is a Cintiq
