It is possible to modify almost any fencing into cat proof fencing; it can be installed using accessories from garden centres and builders merchants.
With the security of knowing other cats will not come into your garden and your cat will not escape onto a busy road.
There are a number of ways to do this depending on your existing fencing etc.
The materials used to put on top of your fence vary.
You could use wire mesh, anti vandal paint, or plastic spike strips, NEVER use broken glass or bottles it is now against the law I am pleased to say. Some use barbed wire or razor wire, this is extreme and not necessary.
The plastic spike strips are not designed to cause harm and will not cut or pierce the skin.
Another option is to use chicken wire this can be put on top of the panel in such a way that a cat could not possibly climb over the top. This is ideal to keep cats either in your garden or to prevent them from coming into your garden.
Using the methods of spike strips or chicken wire mesh is the safest option, it is also the most effective. It is also relatively inexpensive to purchase the materials and to install, I would say if you have just a few panels you can do this yourself easily.
If you have a number of panels it may be better to call in a fencing contractor as he would make this a sturdy installation.