How do you dispose of cat droppings?

You would need planning permission if your fence, including trellis, was more than 2m high. Cats can climb far higher!

 

I think with a fence the usual idea is to put wire netting round the top on a slope leaning inwards, a bit like a prison camp.
This would definitely work with my cats, as they sometimes struggle to jump onto the table, but I'm sure some more gymnastic cats wouldn't find it a problem.
I'm pretty sure that there used to be a company that would come and make your garden escape proof How Cat Fencing Works: Cat Enclosures & Catios | ProtectaPet

 

I used to have lots of cats in my garden
I must admit since I have got Ben (border collie) in April I see hardly any.
He would run at them no doubt, but they just dont come in anymore, its obviously a territory thing that animals respect.

 

Re the bin thing that doesn't seem to work... I read the reviews and for the (high) price it seems pretty pointless as you have to dig a huge pit under it anyway! So I'm thinking about digging the deep hole (I have acquired an excavator so will save effort!), cutting the bottom off a standard plastic bin, and just using that as the sides and lid to the hole.The most important thing is putting down soil and/or compost accelerator after adding poo. After all, that's what happened in old soil water closets

 

You've got a JCB, @CarolineL Attagirl! That's serious gardening

 

Well it's a "small" one @noisette47 - 1.4 ton.
It's ancient but has been spoiled rotten by it's previous owner - done very little but well greased and looked after. I'm calling it "Lucien" - an elderly chap of distinction. As opposed to Molly my Kawasaki mule, who is a game girl, rough as anything but not afraid of work