Okay, I know, you've probably seen this already, but just in case you haven't:
Cat-Ladder: the blog.
At time of this writing 27 countries' cat-ladders were represented. And you know what really made me say "Whoa, Eureka"? The friendly host has amassed a number of links showing further catladders. Take the German CatStep, or maybe you want to buy a Kitty Cat Loft.
Elsewhere on the web that I found all by myself, here's "the cat ladder I made for Brian's cats," with a handy link to aforesaid Brian's cats. And another cat ladder. And "making an IKEA cat ladder" ("My cats are bored.")
Why should there be any area of your house your cat can't access? Don't answer that.