i put the cannons around the castle wall, how do i assign them ? & how do i build a concrete wall ?

You assign cannon the same way you assign any settler to a task, farmer, trader etc.

To build concrete walls go to the wooden floors and use the concrete foundations as walls, I did this in most of my settlements and the Castle to block up the damaged walls.

The same way you assign a settler to crops.. activate the workbench at the castle, point at a minuteman and hit (E) then go to an artillery piece and hit (E) again. The selected minuteman will jaunder over and take up position with the cannon.

For concrete walls I use the foundation floor which you can find in [Wooden structures/floors/foundation] also in the workbench.

Have fun

edit

damn, ninja'd by a freaking Helljumper

Curse you Helljumper and your obviously superior ten finger typing skills!!

If you are playing on PC there is also another option of doing concrete walls by using mods-Homemaker is my personalfavourite one for expaning on settlment building(it wont be same as original castle walls but it will allow you to build proper concrete walls around your settlemnts and much more..)