Gardening mystery of the week.

I have a half butt water feature in my garden and siting on the half but on the small ledge about 3 ft off the ground I have a beautiful Hosta -or rather I did because the slugs have now taken to commando style tactics by either crawling up the butt and into the pot or parachuting down into it. either way me poor old hostas had it!I am at a loss as to how they get in there and how to get rid of the pesky litte critters!