Cats are known for their ability to walk on the narrowest of ledges. They will wibble and wobble a bit, but use their tails as a way to keep their equilibrium and more importantly: to stay up!
Mathilda can do that as well. This is the fence around my garden and you can see how narrow it is. It can't be more than 2,5 cm (1 inch) wide and yet she managed to walk up ànd down: she changed halfway through and came back.