Editing a doorway

Hi, I’m pretty new to Blender and 3D modeling in general, and I could use some help.
I’m trying to edit a doorway that I’ve already cut into the side of the gazebo that I’m modeling without having to start over my modeling of the wall. What I want to do is add an arch structure at the top.
Here’s what I have now:

http://img503.imageshack.us/img503/5363/archwaywirewl8.th.png

This looks okay in wireframe view, but when I switch to solid view:

http://img518.imageshack.us/img518/5169/archwaysolidig4.th.png

The arching section is obviously separate from the rest of the structure.
I made sure as best I could that all the vertices and edges were aligned, but something obviously remains amiss. Can anyone help? If necessary, I can erase the faces of the arch area and start over from a square doorway.
Also, as a newbie at this, I’d appreciate step-by-step instructions, just in case. Thanks!

You could try W >> Remove Doubles in Edit mode. If you placed the new verts by hand and didn’t use Ctrl and/or Shft to snap transforms to grid the may look close enough to the eye, but may not be. In F9 buttons, Mesh tab is another button for Remove Doubles with a Limit value next to it which you could raise, but not so much that it catches verts it’s not supposed to.

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The problem, though, is that as far as I can tell the vertices are the same. Removing doubles had no effect, at any rate.
I notice that when I select the entire gazebo in object mode (using the A key) and then return to edit mode, only the front wall-and-arch section is selected and editable. Oddly, only the curved part of the arch has handles on it.

Probably because the arch is a different object. Combine the objects together.

The weird line you are getting above the arch seems to be caused by that edge above it. The reason you are having selection problems is because your mesh is built in the form of 5 wall objects.

Personally, I would delete the two verts you have on the top left and right corners of the arch, on either side (4 total per arch). The ones that help form a box. Just select them and merge them to the corners.

They may be convenient, but I think they’re causing the problem.

Also, does the line show up when you render?

EDIT: Added the image below.
grab the vert marked in yellow (1) and then select very yellow (2) and hit Ant+M >> Merge at Last. Repeat for the other 6 verts on that side.

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