Selection to Face

I am puzzled as to why I cannot make the selection as shown on the image into a face[email protected]/images/selectiontoface.jpg
Originaly there was edges from the top corners of the doorway to the top corners of the wall which I removed to simplify the geometry they was there because of the way in which I manufactured the ‘doorway’ using the knife tool, are these required to keep/make that side as a face (yes that is what it is). Why Please?
Hopefully I can kill two birds with one stone here. If you have viewed the image you may have noticed the cuboid mesh in the bottom right hand corner, is there a way in which I can use this as a template to punch holes into wherever I want them. Phew!
Many Thanks

Faces in Blender cannot have more than four vertices. Is this what you’re asking?

You can try using Boolean operations to stamp the cube into parts of your mesh.
If you hit ‘W’ and choose ‘difference’ it should work. However, i try to always keep away from using Booleans operation because it can be very unstable.
Just do a search on the net for further info.