LOL. I hadn't thought about the rain. We get monsoon type downpours in the summer which might wash all the dirt out of the pots. I will deal with that when it happens, I have to add soil to all the garden beds every year because of it already.
I do have a garden on the roof, I even have a fishpond up there. Most of the thorny stuff has ended up there although not as a defense, mostly to keep it out of my way. I have Pandanus on the roof and it is pretty painful, it can slice you like a sharp machete. Also there is an unknown vine with razor thorns that I have tried to kill several times. Once I needed its pot and threw it bareroot over onto the neighbors roof to die in the sun. It turned brown but when it rained it got green and started growing again. Then the neighbors did some work on their roof and "returned" my plant! I felt kind of bad and gave it another pot and I water it. It looks nice and soft like a maidenhair fern but it has horrible thorns and will make you bleed if you get near it.
I think bougainvillea is the best defense, it must be overgrown and thick. I certainly would never, ever try and enter anywhere through overgrown bougainvillea. I just gave mine its annual scalping and no matter how careful you are, it always gets you. I don't have mine on the roof and I don't let it get overgrown. Still, right now I am wearing the scars from cutting my few pots of it down to the nubs.
My walls are very high and my problem is not defense but my plant killing cats. If they can't eat it they knock it over. I have to put anything small or delicate somewhere they can't get to it. One of them just got out of the vet hospital because he managed to lean out far enough over the pond to snag the papyrus and eat the whole end of it. This then balled up inside and stopped up his colon! Once they (ahem) got it to come out and identified it, I watched his sister try and do the same thing! I've had to get in the pond and cut the papyrus severely. I also bought them their own square of grass to munch, I have it up on a pot so the dogs don't think it is a fire hydrant! So far, the cats are ignoring it
At least my upload do not get lopsided.
Gremlins 
It's a real pretty bright color to, 
There's always next year !!!
I realized yesterday that if I bought some of the common concrete half pots, made to hang on a wall, I could have the two I have and perhaps some more up on the wall. They'd be safe from the cats and could trail down. I'm going to look for some once carnival is over.
. Quite a few inside walls may also be getting the sun. Don't forget the roof, the rains can water them up there 
. I had a natural forest, small one next to one of my farms. I also had a problem of mice. Took a cat and instead of the mice, it solved the problem of birds in my natural forest. Tried to get rid of it and that thing ran away and became a feral cat. I had to listen to complaints of my neighbors whose birds it also polished off. Never was able to hunt it down. So, no cats.
