Adding a wall
Now we are ready to add walls to prevent the player from falling off the map. To create walls, we will use the same BSP Box brush to create a wall. As we have just added a material in the previous step, you will need to clear this material selection by clicking on anything in Content Browser. This will prevent new geometries from being created using the same material.
Similar to creating the ground, go to Modes | BSP | Box. Click, hold, and drag into the viewport. Set the dimensions of the BSP box as X = 30, Y = 620, and Z = 280. To help us view and position the wall, use the controls to rotate the viewport. You can also use the different views to help position the wall onto the ground. Here, you can see how the wall should be positioned (note that I have panned the camera to view the level from a different angle):
