You can use the Split polygon tool to place an edge from one vertex to another.
You should be aiming to model with 4 sided faces and not Ngons (faces with more than 4 sides) as this will help you keep your faces planar (flat) and avoid the diagonal shadows you are seeing.
If you select a line of vertices and scale them in a single direction to the center of the scale tool they will line up with each other.
Another tip is you can snap a vert in line with another but selecting a vert and then highlighting a direction on the move tool and then holding down V on the keyboard and middle mouse button dragging over the vertex you want to snap in line with.
Finally a better way to have created the battlements might have been to extrude them up out of the wall.

I would suggest that you get yourself a very basic tutorial (some are free for download here ) or work your way through the tutorials in the help files (F1 on the keyboard) to get used to the basic modelling tools in Maya.


hope this helps regards J.S.