How do I measure half the length of a specific part of an object in Blender (3D modeling software)? As you can see in my images, all of my cubes are inaccurately placed on this object and the four corned cubes (labeled as blue) are not placed accurately on the on the corners of this object. Is there a way to accurately space out the difference between each cube, for example I might want each cube to be placed 2 cm apart from each other, and I want all the four corned cubes to be placed precisely on the corners of this object?

How would I do this for this particular situation?
Use snap vertex and an array modifier. By snapping a cubes vertex to the outside edge of the castle turret the cube will be lined up. Then add a array modifier to that cube and set the count to 4. If your math is right number 4 cube will line up with the opposed edge.