Prerequisites:
How to add a Deus Ex style door
Overview:
Once you know how to add a DeusExMover brush, you pretty much know how add windows
and breakable walls too. I'm not going to cover adding a DeusExMover in as much
detail as my door tutorial, so if you don't know how to do that, complete How
to add a Deus Ex style door first. You add windows by creating BreakableGlass
mover brushes, which is a subclass of DeusExMover. Though it's called
"BreakableGlass", there's a simple way to make glass unbreakable too.
Actually, the first step should be to make sure you have cut out an opening for your window. Make sure you're actually connecting two "cut out" places, of course. I tend to use the 16 unit grid so that the glass window itself will fit perfectly.
Shape the red selection brush into the size of your window and place the brush
so that it is floating inside a room, just like when making a door. Make it
1 unit thick for this lesson. Windows that are 1 - 4 units thick
seem to look the best. Hit the Add Brush to World button: 
,
and texture it. The CoreTexGlass texture library has several you can use.
A good one to use for a generic window is the first one: 07WindOpacStrek. The
texture looks black in the browser, but in the game a faint texture will be
visible. Just like any other mover, you can't change the textures once
you actually create the mover brush.
Before going on, be sure that all of the surfaces of the brush are set to "Translucent". Do that by right-clicking on one of the brush's surfaces and pick Select -> All Adjacents. Then right-click again and pick Properties, then click the Translucent option under Effects.
Now size the red selection brush so that it is a bit bigger than the brush
you just added. Be sure to check all 3 2D windows. Hit the intersect button:
and the selection
brush will be perfectly sized to your window brush. Make sure Classes is selected
in the Browse drop-down and expand the following items: Actor, Brush, Mover,
and DeusExMover. Select BreakableGlass. Now press the Create Mover button: 
.
As with any movers, at this point, you will actually have THREE brushes in the same place: the red selection brush, the blue "Addition" brush, and a purple "Mover" brush. Move the red selection brush out of the way, and click on the purple brush to select it. Move it somewhere, and you'll notice that your original brush is still there. De-select the mover brush and select the blue brush. Delete it (either hit the Delete key on the keyboard or right-click and select Delete). Now hit F8 and Rebuild Geometry to get rid of your original brush in the 3D view.
The purple brush that is now in your map will only show up as an empty wireframe. However, it will show up fine when playing the level.
Make sure the grid is on, and move your window into the opening that you created. NOTE: I originally thought that you had to move the brush's KeyFrame 1 out of position, but that's not the case.
Don't want a window to be breakable at all? Easy, just right-click on it and pull up its properties. Expand the DeusExMover item and set bBreakable to False. You can also play with the other properties to customize your windows (such as making them stronger or weaker), but the default values work well.
When testing out your breakable window, it's helpful to make sure there's a
crowbar or other weapon nearby.
Rebuild your geometry, save your level, and break something!