If you are renting, run the ideas you get past the landlord first to make sure that you are within the stipulations of your contract or problems might ensue. P.S. Are you allowed animals in your flat and if so is there a yearly fee for each animal attached to your rental for this? Make sure before you approach the landlord about the aforetomentioned.

Wouldn't have kept my younger moggie in--he thinks he's an extreme free-climber! :)

Willow screen fencing or reed/bamboo, secured into the ground (willow fence panel) or attached to the railings. To try to prevent your cat getting out on top, that's more difficult, can't think of a way you can do that, you'd probably need to use a strong wood batten and attach rigid trellis sections or something and secure that in some fashion - probably with fence posts on the inside and the wooden batten strung between them. I doubt any of this will keep the cat in though - in my experience, any healthy cat, if it wants to get out of the garden, will easily find a way. Those claws are pretty useful for shinning up verticals...