My hamster loves it. I think its the dark spaces and tunnels that it likes.
Step 1: Materials
You'll need LOL!
Lots of Lego!
I was luckey enough to get my dad's classic red and white bricks, baseplates and windows.
Step 2: Ground Floor
3 base plates long, 7 bricks tall. Also put the stairs in at the end.
Step 3: 1st Floor
Put base plates on as shown and build up. Leave out a panel on the roof so it can get out onto the roof. More floors can be added but i didn't have enough bricks.