Hey Blender Fellas,
I have a serious problem, Im wokring on rebuilding a castle. Simple I started from a cube, Now I need to make a circle shaped Window i dont know how to make the point on the surface of the Cube in Circular shape. Please Help. I need to make it from the same body So I can extrude it in.:(:(
For 2.55
Select face you want to cut a hole into. (in face selection mode)
press <w> select subdivide, for this example, set number of cuts in the toolshelf <t> at the bottom to “3”
Select the center face.
Press <shift+s> and select “cursor to selected”
With the face selected, go to “view>align view>align view to selected>top” - now your camera ZX plane matches the face wich is important for the next step.
press <x> and select face to delete the center face.
press <a> to select all faces and press <w> again and subdivide again, this time set the number of cuts to 4
press <a> again to DEselect everything.
change to edge selection mode, hold <shift>+<alt> and rightclick one of the edges that form the inner opening now. this will select the edgeloop forming the inner quad representing the opening (“rectangular window”)
now press <shift>+<alt>+<s> (to sphere) and type in 1 and enter. this will align the selected edges spherical around the cursor, projected to your view.
that is why you need to set the cursor to the center first and you got to have the plane in the xz-plane of your view.
Now you should have a plane with a spherical cut in it.
For the future, use the tools at hand to gather information 
Google… “blender cut round hole”
Forum search… often asked, often already explained.
To add a circle, just go Shift-A and add a circle while in edit mode, then place it where you want it and connect the edges.
Or, you could extract+scale the wall to make a square window, then subdivide it with smooth, which will make it round.