How to make an arch in a model w/o Bezier Curves and w/o adding separate objects? I would like to curve a line in order to make a medieval door.

I'm talking about this curve:



I can't understand and I can't find any tutorial that tells you how to make this kind of curves, also I would like to not use the Blazer curves.
I'm trying to find something easier because I would like to know how to use less time in order to make curves.

I saw that you can do it using the array modifier but it can't be used on edit mode, should I add another object? Is it a good practice to add objects to modify the main one?

This is where I'm trying to make this door.. 
Thank you and sorry for wasting your time, I'm just a beginner..

Edit: Why my question is not a duplicate of another -->

These are different questions, I'm not asking how to do it with a single object, I'm asking how to do it if I have a building, because the modifiers don't work if you are in edit mode, so I'm asking how to make a arch if I have for example a house and I want to have a arch door or arch windows. It's a different thing.. 
I would like also to know if it's a good practice adding new objects to another one, because using this way I can for example make a cylinder and remove half etc.. 




Edit: The solution from @moonboots worked! Thank you

Result:





Thank you!
I've kept your topology but no need for so many edge loops. Anyway, you could use the Bridge tool:


First, tilt a bit the top faces of your left and right columns.
Press W > LoopTools > Bridge, or W > Bridge Edge Loops.
Play with the parameters on the bottom of the Tools panel.
Fill the hole the way you want to finish the work. Here are 2 ways to do it, maybe there's a better one though.