A tip from the tv program "My Cat from Hell":

The host maintains that a cat can and will climb a fence of any height, but will be defeated at the top by a strip of wire netting set at a 45 degree angle pointed in and down. This blog is the chronicle of a guy's struggle with his cats. One note about plastic netting: I had a cat that lost a leg because he tried to climb plastic netting in the garden (designed for peas to climb). He got tangled half-way up and hung from it overnight, cutting off all circulation and eventually killing his leg. I'd just use what is called "chicken wire" here in the USA.