Ok so I have this made from half a cylinder and I have cut out the faces were I want a door to be:
I want to create something like this:
You see how the doorway is part of the cylindrical shape and has its own arch? I would then extrude the edges when I have created the arch. I first tried manually adding edge loops but it just did not look right. How can I create a nice curved arch from the square doorway in the first image?
The easiest way would be to drop a pipe (or tube, what it is called), then WITHOUT going to face, edge or whatnot mode, as a polygon primitive, you can set its inner, outer radius etc. Do this in otrho side mode, or anywhere that it faces you. If you are happy with the way it looks, keep only its faces that look at you, delete all the back faces and the side ones. Now you have a poly ring left. Delete the lower half, or whatever you want, just keep it symmetrical, and extrude its two side edges down.
Another method is using extrude along curve that is a wee bit more complicated.