"Vertex manipulation? what's that? can't i just use carve tool in hammer?"
Vertex manipulation is used to drag something i call "brush points", for example cube-shaped brush consists of 8 brush points, or vertexes.
8 vertexes -- "brush points" -- in a cube
vertex manipulation tool
can be used to manipulate brush point positions, so you can make some complex shapes when you have multiple brushes, this tool will work on any brush, not just on a cube.
Great now that you know what vertex manipulation is, let me show you why carving is bad in hammer:
Notice how carving creates so many new brushes just to make a simple shape inside the original brush?
not only is vertex manipulation better for game performance & compiler, it also looks cleaner in hammer.
And vertex manipulation can save on brushes too!
Carved doorway (incl. floor): 15 brushes
Carved doorway (incl. floor): 11 brushes
Now, lets make one step by step!
first off make a brush for your usual block-y doorway.
then lets create a cylinder brush, i usually use 12 faces on it. You can guess the size or rotate your door brush by 90 degrees and then back. I rotate it so its simpler.
try to use sizes that are 1:1 for the cylinder (128x128 in this case) so it looks better, now rotate them both (or cylinder only if you did not rotate the door brush) back and re-size your door brush so that its top is in the middle of the cylinder
now, lets chop up the wall manually:
now this is where the fun part starts at, lets start using our vertex manipulation tool, but first clone a part of your wall so we can manipulate that, position it either on the right/left top corner and press "vertex manipulation tool"-button (or press SHIFT+V)
You'll probably notice that your brush now has these boxes and lines drawn on it, that means its in vertex manipulation mode, now lets first move our top right point to the top left point and join them (it'll ask you, just press yes), do this by using a side view and simply click&hold then move the point, don't worry it will grab them both
now lets reposition our bottom points according to our cylinder brush (you see, we created it so that we have something to base it on), you might need to go to the smallest gridsnap depending on your cylinder brush size/face count. After you have finished doing that just simply take your selection tool and deselect the brush.
now repeat this process few times for the side you are working on
Great! now we have half of an arch, lets copy&paste it and rotate it so we have the other half:
you might notice that inside the arch, your textures are skewed and not probably aligned correctly. lets fix that by using this neat little thing in hammer which nobody seems to know about:
First off take your texture application window and select the wall on either side
now just simply hold ALT and right click the brush face you want to align based on your selected brush, do this for all the faces on the side where the wall is (so 3 faces for left and 3 for right, kinda hard to explain), and then select the opposite wall and align those faces too. now you should have perfectly texture aligned & non-carved arch!
Hope you enjoyed my tutorial on how to make arched doorways!
I mainly wrote this because i've seen too many people trying to do this by carving the shape.