Carving is probably the most abused function in level editing. Regardless of the editor, subtraction is akin do doing surgery on your level; it has to be carefully thought out beforehand. There are two main problems that a bad subtraction will cause. 1 - The mess of small clustered brushes often cause compile time errors. Nothing is worse than a cryptic error from QBSP. 2 - Unmanageable brushes. Once you've created your doorway you may find you need it to be bigger. If you don't have a neat set of brushes to work with increasing its size becomes awkward. The best way to solve these problems is to minimize the area that you are carving. Consider the following two pictures of a cylinder subtracted from a wall:

In the first picture the cylinder was simple placed in the center and subtracted. If the wall was larger those fractures would extend to the end outer edges of the brush. This is defiantly a problem if you wanted to change the size of the wall later. In the second picture the subtraction area was constrained by limiting the size of the brush that was subtracted. As you can see this creates a much neater subtraction and it would be very easy to manipulate the surrounding wall.

In the next example we'll carve three arches through a wall while keeping the resulting brushes manageable. The doorways will have a 64 unit wide opening and a 32 unit wide frame (128 units wide in total). Multiplying this by three means we need a 384 units long wall. The doorways should feel grand so we need them significantly higher than the player. Twice the players height is 128 and add 32 units for the frame means 150 units tall.

Now, here is the example of what NOT to do. This is the #1 most common error people make when editing and what most people mistakenly do. First a brush the same shape as the doorway is created. We grouped a rectangular brush with a cylinder on top to make the 'arch' and then simply stuck the brush in the appropriate areas and subtracted. The result is a number of odd fractured brushes that would make it difficult to manipulate and even to texture.

Now a better way of going about it. The first step is to begin by making the original brush into three smaller brushes.

The next step is to minimizing the carving area. We build a brush that is the same height of door including the "arch". Then crave this so it leaves us with the opening for the doorway (first picture). Then place a brush along the top of the doorway the size of the arch portion of the door (second picture).

Now to create the arch. We use an eight sided cylinder. The cylinder is exactly twice the height of the upper brush - this is to ensure that the vertex points line up otherwise we'll get unwanted brushes after the subtraction. In the first picture we place the cylinder in position to carve. After the carving (second picture) we're left with a neat doorway that is easily manigable. Compare this to the previous picture of 'The Wrong Way'. See how we've eliminated the 'shattered glass' look? If you discover that shattered glass look after carving then you'll probably want to find a way of limiting the effected area.

Now that we have one complete archway we delete the other two wall section and copy/paste two copies in. These arches can be cleaned up even more. In the second example we've removed all the redundant brushes and finaly we stretch the existing brushes to take their place. This leaves us with a perfect set of arches that could be put in out prefab for later use in other levels we might design.

Many thanks to Somberfire of the ELM design team for the steps and examples of proper arch carving.