Link to the blender file if anyone need it : https://mega.nz/#!BxBkVA5b!Xz0vgBDgfhW32xzLA_DX5Xp-r9dFMtxl2wbwyY0Km5U
A face is a polygon. A polygon is an array of vertices.
The way it is interpreted is:
- Make a first triangular part with vertices 1, 2, 3
- Then make another one with 2, 3, 4
- Then 3, 4, 5
This does not allow to have holes in it except if there is only 1 hole, or in some special cases for example if the holes are triangular themselves (at least, I think, this is surely a complex topology problem...).
So, my though is that you can dissolve only if there is a path of triangles "calculable" all along this polygon. And I don't think this is the case here due to the 3 square holes. Or if you dissolve, the polygon will cover/overlap the windows.