As I said before, that was the star fort unit, included in the Designer's Mod, it has those units on the wall.
The night, you can easily make it yourself, by going into the editor and change the lighting.
But, it is possible to put regular units on regular walls too, this is either done by: modding the unit so it flies, use the javelin trick (there is a guide about that somewhere up here, I think), or by making "spikes" of ground beneath walls, then placing units on the spikes.
None of these shots are impossible, nor created by ES.