Also, if you want to have a specific arch texture assigned to the wall, you have to follow another procedure which is actually pretty easy and is a good way to become familiar with the extremely useful 2D shape editor. Basically here are the steps:
1) choose the texture you want and export it as a .pcx file --- i usually just use the texture name as the filename. BTW, you need to export it because the 2D object editor, which you will use, doesn't accept pcx files for some reason.
2) open up Microsoft paint and open the file. Save it as a high-color bitmap (bmp).
3) back in UnrealEd, open up the 2D shape editor (its under the file "window" at the top of the screen.)
4) click on "image," then "load image." Choose the new .bmp file you made and it should appear on the screen.
5) using the editor, make your shape, "extrude" it (its under the menu "loft") and you will have a ready-made door for the texture. If you want an arched hallway, simply make a block in your level first, and subtract the arched shape.
If you need help with the 2D shape editor, (I know it IS kind of annoying and buggy /~unreal/ubb/html/wink.gif )
www.unrealed.net and unreal.gamedesign.net have some great tutorials.
Good luck!
Alex