[Sphynx]

This is the second most simple method of creating a window or portal in a wall. Again, we are talking about regular shapes, so we can use the built in tools in most cases to make our cuts.

1. I start with a wall and added a second object in the form of another 32 sided circle to represent where we want the hole to be. If everything was perfectly aligned, we could just join the two objects then select points to create into polygons, so we are going to assume that we don't want this - I've rotated the circle a little so that our hole is going to be an ellipse, not a perfect circle.
2. To get a reasonable object that we want as a window, I've selected all of the points of the circle and hit [shift-f] to generate a surface within the circle. We do this as we want to extrude along the circle's normal - we do this next by hitting [e] and extruding the region to get a cylinder (you could just generate a cylinder from the start, but this is teaching skills ).
3. I position the cylinder so that intersects the wall were I want the elliptical window to be.
4. I select the cylinder then the wall, then press [w] to reveal the boolean cutting options. I select [Intersect] so that the cylinder cuts a depression into the wall surface. This should leave a new (unselected) mesh with the depression, with the wall and cylinder still selected. I don't need these now, so I can move them to another layer for safety, or just delete them using [del].
5. Selecting then editing the new mesh, use vertex mode to select all of the back ring.
6. These vertices are not required, so I delete them. As we are deleting the vertices, the lining polygons back to the wall can't exist and so delete as well.
7. At this point, there is nothing more that I need to do - these are simple objects, so the intersection was quite a good operation and the only 'clean-up' that was needed was to delete those back vertices. Depending upon the nature of the wall and portal, you may need to perform more clean-up.