I made some changes in the latest revision that will help with this, but in the XML spec, the Buildable attribute indicates if a tower can be built on it. So you can mark a cell as None, and then make Buildable="false".
All of the changes are in revision 69610: http://scripttd.codeplex.com/SourceControl/changeset/changes/69610
So if you want to apply those changes to your source, check the diffs for each file to see what has changed. Basically I removed the exception when building if you had a special cell without a name, and then I added a check inside Grid.cs, in the ApplyMap
method to apply the Buildable and CellType attribute to the normal cell if it has no name, or treat it as a special cell if it does.
Now you can specify an obstruction where a creep can pass through using:
I hope this helps. :)
<Cell Type="None" Buildable="False" X="whatever" Y="whatever" />