My parents had that problem and they noticed that the cats had one point of entry, which was jumping from a neighbours shed onto a railing and then down into the garden. My parents then cut up a length of plastic piping the lenght of the railing and clipped it onto the railing, thus making the railing super slippery. The neighbourhood cats tried getting in once or twice but soon learnt their lesson and stayed away.
Really like the water bottle suggestion, I'm going to try that myself (as my parents solution can't be done in my garden), it seems like I would work.