We are going to make an archway for a specific texture. The texture I have chosen for simplicity is Door2B in the Ancient package.

The first thing you should therefore do is load this package and select the texture.

For the room all we need is a 512x512x512 box subtracted. Then add a 256x256x16 cube with the arch texture. This should give you the picture on the right.

Then load the 2D editor.

To set the texture as the background choose File->Image->Get From Current Texture. This will load the texture into the background of the editor.

N.B. Many people have reported to me that this particular action crashes UnrealEd. I have no idea why - the only solution is to export the texture and use the File->Image->Import Texture option instead!

The simplest way to create the cut out section for the door is then to move the 4 corners of the square to the sides of the black area and make top segment a bezier curve. If you then move the bezier points to just above it creating a lovely curve you should end up with something looking like that in picture 2. The green marker is the pivot point of the created shape which can be moved and the red marker is the centre of the shape and cannot be moved.

You can increase the smoothness of the top line by going to Edit->Segment->Detail and choosing say 10 (I think the default is about 4).

Once you are happy with the shape click on the Extrude icon or choose Process->Extrude and enter 16 for the width (or whatever width you made your wall). You will then have a brush in the world which should fit exactly the hole you have.

The finished article should then look like the image below.