Securing a backyard animal / cat shelter fence from escape?
Any fence that will even stand a chance of keeping a cat in needs to be like 8 feet high. Depending on how adventurous they are, they'll just jump on top of the arch. 

And if they're declawed you shouldn't be letting them outside unsupervised anyway. 

But w/e... buy more metal posts, bend them at a 45, 1/3 of the way up, and attach that third (as elsewhere with something better than plastic zip ties) to the existing posts. Make sure you get the homeowner version and not the industrial, or you won't be able to bend it.