I will soon be starting to build a cat tower for my 3 cats as well. Something to take note of though......in doing my research to find enough sisal rope (I needed over 300 ft.) I discovered that the sisal rope that is generally sold in stores (home depot, lowes, local hardware stores) is oiled and has pesticides on it. A no - no for our kitties IMO. I got this info direcetly from a rope manufacturer. If you decide to put sisal on your cat furniture, make sure you get un-oiled sisal.
Good luck with your scratching post!