I hadn't originally planned to write an instructables, so I didn't take pictures of every little detail. I've also not written an instructables before - but hopefully these instructions are still good enough. :)

I wanted to get a cat tree but didn't like the ones you can buy - so I built my own. This cat tree was built for a cat who loves to climb things, so the tree is fairly high and all the platforms are facing towards the wall so that the cat can climb up the post.

The cat tree is 180 cm high and all platforms are fixed to the wall. The lowest (large) platform is 60 cm above the ground, the other (small) platforms are attached every 30 cm, i.e. at 90, 120, and 150 cm, and on top (180 cm). The small platforms are 20 x 40 cm, the large one is 40 x 40 cm. These dimensions work well for a medium sized cat (5 kg).

Materials:

square edge timber (2400 x 44 x 44 mm)
plywood (16 mm thick) cut into four 20 x 40 cm and one 40 x 40 cm pieces [for those in the UK: you can buy this at B&Q and they will cut it for free]
sisal rope (10 mm diameter)
angle brackets (I used lightweight 100x100x20mm brackets for attaching the platforms to the post, and lightweight 40x40x15mm brackets for attaching the platforms to the wall)
screws and wall plugs
double-sided tape
optional: carpet underlay
carpet
timber or plywood for the base (minimum size 450 x 50 mm)

Tools:

saw
hammer
screwdriver
drill
staple gun (heavy duty)
Stanley knife
measuring tape
spirit level

Step 1: Prepare the Posts

Cut the timber (240 cm) into two pieces: 180 cm and 60 cm.

Attach the brackets to the posts, using reasonably long screws (something like 4x40mm should be alright). Drill holes into the post before putting in the screws.

60 cm post: one bracket at the bottom and one at the top.

180 cm post: one bracket at the bottom to attach the base, 4 brackets along the post to attach the platforms (at 60, 90, 120, and 150 cm), and two brackets at the top. First I only used one bracket for the top platform but it was too wobbly, so I squeezed in a second smaller bracket (after I had already attached the rope to the post). I would definitely recommend using two brackets for the top platform.

Step 2: Wrap the Rope Around the Posts

Attach the sisal rope to the post by putting a screw through it.

Attach the rope on three sides, to make sure it doesn't slip off the post. On the fourth side it is held in place by the bracket.

Start wrapping the rope around the post. I found it works quite well to turn the post while keeping tension on the rope. Make sure the rope is wrapped around tightly, hitting it with a hammer every few turns helps to make it more compressed.

When you reach a bracket, attach the rope with a screw directly underneath the bracket - that way the screw is out of the way and the cat won't get the claws caught in it.

When you reach the end of the post, attach the rope on three sides of the post again (the same way as at the beginning).

Step 3: Prepare the Platforms

I used 16 mm plywood for the platforms, but other materials (such as MDF) should also work.

Optional: Cut carpet underlay to the same size as the platform and attach with double sided tape. Some sort of foam could also be used instead of underlay.

Cut carpet to size, slightly larger than the platform (additional 2-3 cm on each side). Attach carpet with double-sided tape.

Staple carpet to the underside of the platform. I used two rows of staples to make sure the carpet is properly secured. Leave space for attaching the brackets. On the side of the platform which will be facing the wall I only attached the carpet on the side of the platform and not the underside.

Step 4: Put Everything Together

The platforms are fixed to the wall with small brackets. First attach these brackets to the platform, and then attach the platform to the post. Each platform is attached with two brackets, except for the top platform which only has one bracket.

I assembled the cat tree in the following order:

attach the three middle platforms to the tall post
attach the top platform to the tall post
attach the bottom platform (40 x 40 cm) to the tall post
attach the short post to the bottom platform
attach the base (I used some timber which I had lying around, but a piece of plywood would also work)

Attach the cat tree to the wall. Each platform is attached with two brackets (except the top one, it only has one). Make sure the screws fit through the holes in the brackets before drilling holes in the wall. Use appropriate wall fittings - I used 5 x 50 mm screws and 45 mm wall plugs. The platforms bend a little bit because they are only attached on three sides - but not much, and they are more than sturdy enough to hold a cat.