I have 8 litter boxes, scooped twice daily. They have a whole room (extra bedroom) for the "litter box room". No other furniture in there. Yes, I know I should put litter boxes in different rooms, but that is just not possible right now. It definitely is territorial, as they can see neighbors' outdoor cats in our yard, and then pee on the window, etc. They are gradually losing access to the whole house because of this. My bedroom is now a "cat free zone", as is the family room and laundry room at the other end of the house. The hall bathroom door is kept closed all the time, as they have peed in there so much it has gone into the floor boards. I am going to rip out that floor this spring, replace the subfloor, put ceramic tile down and seal all the edges with caulk.
They are at their worst when I am out of town--I don't know whether this is that "guide her home" thing or not. When I'm gone, they pee in places that they never do when I'm home.
I really am at my wit's end, but do not have the heart to just take them out and have them killed! One of the problems is that one of the cats is brain damaged (cerebellar hypoplasia) and has difficulty walking, and is bullied by the others. He's not the worst offender, however!
Sorry Citygirl! I didn't mean to hijack your thread, but as you can see, this is a subject of great concern to me!
Seriously, the Catscat mats DO work! You can cut them with scissors to fit the pot, and that's just about the only place my cats DON'T pee! You can buy them at gardeners supply stores online.