Thing is, you have to have the tools to cut it to shape, and we don't know if feline is a gal or guy, but some upper body strength necessary to drive in rebar...
If we're talking a little gal, the cement might be the easiest way to go.
Haha. Feline is a not-weak-not-powerlifter gal with no strong male friends.
I like the idea of the railroad tie, and I even have some in the vacant lot behind my place, but cutting it would be a beast, and (naturally) it’s not a clean angle. There’s a minicurb under the main one that’s uneven and broken away, which is why they added the gravel.
I’m thinking concrete is my best bet.