Run Radiant and open the map.

Select grid size 5 and draw a brush as shown.

Give yourself the 3D view that will show you what's going on in the corner.

Depending on what other brush manipulations you've been doing, you may or may not see the brush in front of you.  Let's assume you don't.

Click Curve\Bevel and see what you get in the 2D view.  The brush has been converted into a curve called a patch.  You still select patches just like a brush, ie shift+click.

Do a Z-axis rotate 3 times so that the curved line fills the gap between the walls.

Ctrl+tab so you can position the curved section to fill up the gap from floor to ceiling.  You'll need to change to grid 4 to make the fit.

In the 3D view you'll now see some faint lines of your curved surface, but no texture, so you can see through the new brush.

Click Curve\Matrix\Invert to flip the drawn face to this side.

We will give the new curve the matching wooden texture of the rest of the wall.

Press shift+S to bring up the Patch Properties window.

Shift+S for patches (the name given to this type of modified brush) is like S for the faces of regular brushes. BUT: if you accidentally press shift+S when you meant "S", or "S" when you meant shift+S, you will very likely crash your PC.

Click Natural.  This will look better, but it may be upside down.  If so, click the up arrow Rotate Step four times.

Press ESC.  It should now look like this:

With a texture that has no discernible pattern, like concrete or metal, you won't see the join.  With this panel effect, we do see the join.  It probably won't be noticed by anyone: did you notice the bad join in any of the inner corners?  They are probably there but not noticed.

We can either cover the join with some cosmetic furniture or trunking or similar; or we can adjust the positioning/stretch of the texture to try to get a better fit.

Select all the inner room wall faces, other than the curved bit, but including the faces around the window and door, and click the wood_test texture again.  This makes the texture run properly from one face to the next, but it also highlights what happens when brushes don't sit squarely at Z height intervals of 128.  Because our wall brushes are up a notch, the textures are vertically wrong by a notch.

We correct that by pressing S and the up arrow V Shift 4 times and then click Done.  Press ESC.  So alright, the wall textures are all run smoothly, but the curved texture still doesn't quite fit.

Select the curve patch, zoom in in 2D and press V (Vertex tool).

Click on the top left green dot and drag one notch to the left.

Click on the bottom right green dot and drag one notch downwards.

Press ESC twice.  We've now got the curve probably as good as we're going to get it.

Finally we want to eliminate the wasteful face drawing that is behind our new patch.

Still in overhead 2D, select the north wall and cut it (X tool) where shown.  Use Shift+Return to keep both parts.

Press ESC twice and select the side wall, and cut it also where it meets the edge of the curve.  Press ESC until everything is deselected.

Now select the patch and Hide it.

Select the wall faces that are obscured by the patch, and caulk them.  Press ESC.

Then press shift+H to reveal the patch again.

Save, compile and go and see how it looks in ET.

Draw a brush as shown, then Select Complete Tall and Hide them.  This just makes it easier to see what we're doing in the room.

Let's put in a couple of uprights to be the arch supports.

Draw the brush as shown, caulk it and make it detail.

Get a side view and make sure it goes from floor to ceiling.

Select the 3 visible faces and give them the church_c01dm texture.  Press ESC.  Get the 2D overhead view and then duplicate the brush, mirror it in the X-axis (x-axis flip button), and move it to the opposite wall.

Press ESC.  Now we'll put in the connecting ceiling section prior to creating the archway.

Draw a connecting brush as shown and caulk it and make it detail.

Get a side view and shrink the brush up to a narrow strip at the ceiling.

Apply the brick texture to the 3 visible faces of the new brush.

Overhead view again in 2D and draw a brush as shown.

Curve/Bevel to turn it into a patch, then rotate it 90 degrees in the x-axis (x-axis rotate button).

Reize the brush until it's the same width as the columns.

It's the wrong way round for this side of the archway, so rotate it twice in the Z axis.  Then ctrl+tab twice so we can move it vertically into place.  It should be placed as shown here:

In the 3D view we can see the texture is on the inner face and we want it on the outer face: Curve\Matrix\Invert.

Now we can also see that the texture is oddly stretched.  Press shift+S and click Natural, then rotate it 90 degrees by clicking the up arrow rotate step twice.

Now to fill in the gap: Curve\Cap Selection\Inverted Bevel\OK.

We end up with a func_group consisting of the curve and two caps to plug the gap.

With the group still selected, get an overhead view, duplicate the group, mirror it in the x-axis and move it to the other side of the archway.  Press ESC.  It should now look like this:

When you press shift+H to reveal the hidden brushes you will see the light is sticking through the arch wall, so move the light so that it isn't stuck in the wall.  It is shown in the next picture with models filtered out so you can see where I moved it.

Duplicate the light and drag the new light down to the middle of the new room area.

Save, compile and test the look of the arch.  Backup your work if you haven't, in case it all goes pear shaped later on.

Strictly speaking we should cut the upright columns and caulk the faces that are obscured by the arch patches.  Let's take that as read and move on :)