First of all, there was the idea to take in two kitties from the local animal shelter. To avoid to much damage to furniture I wanted to have a scratchpole for them and also a place, where they could hide.
If you are a cat-owner by yourself you probably know the ugly plushy cat-trees which are usually sold in shops. For me, this was an absolute no-go! So I headed out in the internet looking for alternatives an found this:
http://www.moderncat.net/2008/09/21/custom-kitty-castle/
and this:
http://www.moderncat.net/2008/12/29/kitty-catsle-copycat/
I immediately thought, that underneath the "balcony" I should be able to integrate the scratching pole and started sketching.

Step 1: Materials and Tools

As the castle (or catsle as it's called on moderncat) should match the bookcase next to it, I decided to build it in laminated 16mm birch plywood in Germany commonly known as "Siebdruckplatten" (screen printing panels?).
The joints I made with simple wooden bars (20x20mm), screwed into the corners. One could also use metal angle brackets or if you have the tools on hand, you could also use a biscuit-joiner, Lamello or similar (I would have preferred this).

As other tools you will need:

jigsaw
drill with necessary bits
screwdriver
carpetknife

materials / partlist:

wood 16mm:
- 2x: 1600x470 mm (back and front)
- 2x: 1600x600 mm (sides)
- 2x: 470x568mm (top and lower floor)
- 1x: 568x800mm (middle floor and base for pole)
some carpet of your choice
scratching pole (mine was the cheapest I could find, plus a spare piece,screwed together with a fitting rod)
screws
glue

Step 2: Building

The wood I bought cut to the right size already, so I only had to make the cutouts an the drilling. I think I don't need to explain this part in full detail because it's pretty straight forward: make the cutouts and assemble according to sketch. Best do it from bottom to top and front to back and put the carpet on the floors before(!) you fix the backwall!…
Just make sure, that you do the larger cutout for the "balcony" as proper as possible, because you will need this part as the bottom for the pole!
The measurings are all in the pictures. If you need them in a larger size go to my flickr set.
According to the hight of your pole, you might need to adjust the height of the balcony-floor.

Step 3: Set Up and Let the Cats Enjoy

When assembled you only have to put the castle in your livingroom an hope the cats will accept it. Some nice and cosy blankets or cushions and some sprinkles of catnip might do the trick…. ;-)