Well here we learn how to create a wall in valve hammer editor.
1 - Open the program, click New.
2 - Now click Block Tool [Shift + B].
3 - Now click on the of the square camera, click and make the size, height that you want. On the top (Top (x / y) you change the width of your wall, the screen below (Side (x / z) you change the height of your wall. After leaving the wall as you want, with clike right click in the area of the wall and click create object.