Cat Proof Yard Fences

We had this problem, where 4 cats were having a turf war over who owns our backyard.

There was one neighbour's feral cat in particular which just hated all other cats for some reason, and would fight viciously with every one in sight as soon as she was let out. Unfortunately, one of our cats seems to enjoy the adrenaline rush of facing an opponent. So it was not a good combination to have these two cats outside near each other.