The following image sets out the environment that we wish to create.

This image shows us the room that we want to have.

This image shows the exact same room in the overhead 2D view.

As you can see, it requires three brushes to create this environment. The proper order is as follows:

1. The first brush created is the room itself, giving the player room to walk around in.
   

2. The second brush is the pillar itself, because it must be added to the space that the first subtractive brush creates.
   

3. Finally, the hole in the pillar must be subtracted after the pillar has been put into place.
   

Sounds simple enough, but lets see what happens if the order was wrong. Changing the order of the hole in the pillar to the first CSG Operation will result in the following (Following the way that the Unreal Editor creates the environment, it would do the following based on the order of brushes.):

1. Finding the first brush (this time, it is the hole inside the pillar) it subtracts the space from the solid space. This leaves us with a tiny room.
   

2. The next brush is now the original room. Because the room is also a subtractive brush, the full sized room is now subtracted. This leaves us with a larger room, with a smaller room inside it. At this point however, the smaller room is not visible, nor does it have a purpose. An area cannot be subtracted twice, hence only the larger brush has any consequence as it is second in the order.
   

3. Finally the editor comes to the third brush. This is the pillar, which is then added to the larger room. Even though the hole in the pillar has been subtracted, the pillar is added after the hole has been subtracted, hence it overrides the hole. We are left with a room and a solid pillar in the middle of it.
   

This image shows the same room with the order changed as described above.

This image shows the same room, after the order of the brushes has been changed. It is noteworthy to see that the brush that makes the hole in the middle of the pillar can now only be seen where the pillar does not cover it, unlike the 2D image above, where the sides are seen over the top of the pillar brush.

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