Don't think snakes would discourage the cats. In my last house I had a beautiful grass snake which lived in the bank behind the ditch. It was about 5ft long and hissed loudly, I called it Hissing Sid. Poor Sid was always being prodded and poked by my naughty Siamese. Then I noticed four baby grass snakes in my pond, sweet little things. Of course, before long, Smokey Minty and Murphy had caught the lot and brought them indoors, I was gutted, as I love wildlife.