Caution: Lots of pictures in this post. The construction of this thing took a while and I tried to document the process.
The actual construction of this was slowed down some by having to wait for glue to dry. If you want to see the the design portion or my idea post on this, it can be found here.
Overall, here's the process I followed for the construction. I started with the base. Once I had the actual footprint of the model, I built my walls up from that. Once I had the basic/complete walls cut out, then I went in and created damage along with adding all the smaller surface elements to make the building look believable.
With all the walls "filled out," it was a matter of assembling the pieces together, reinforcing key areas and adding some smaller interior details like trim and catwalks and such. The further I got along in the construction process, the more custom the measurements became. When I first started, I could build things based on measurements and what I knew the distances would be. As I went along, I had to measure each thing across all sides to get an exact fit to the piece. This slowed the process down even further, but everything fits together in the end.
And if you look carefully, you can see my one Chaos Marine I used for scale purposes. It gives you an idea of the sheer size of this thing.
This thing was built to play on. The catwalks are wide enough for 40mm based models to move on. The steps up to the altar area are wide enough for a 60mm (dreadnought) based model to move up without trouble. The hole in the side wall of the building is wide enough for a Rhino to fit through. Actually, I think a Land Raider will fit through it as well.
I also was careful not to line up all my openings in the walls so there are places where LOS is completely blocked and places you can see, but the enemy obviously gets a cover save.
In the end, I tried to make this a piece you could use instead of something that just took up space on the tabletop that you moved around.
Part 3: Pics of the completed model.