Well it I had 100,000 in my garden we would be knee deep in the nasty little things.
I have found that a rough base around the plants keeps them at bay as it irritates their undersides.
Eg. Sharp sand especially if it is unwashed as it contains trace amounts of salt so they avoid it, washed is almost as good.
Bark chippings are not bad if small, the coarse sawdust from using a chainsaw is very good, but need replacing each year as it rots. Chippings from an industrial chipper (the sort councils and tree surgeons use) seem to be the best as they are good and rough and the right size.
Also seek out and destroy the damp areas where snails and slugs reside.
Encourage Hedgehogs into your garden, they are quite common even in towns and love slugs, we feed ours (3 of them) the left over cat food to encourage them in and provide "passageways" under the fences so they can get in & out (the food is placed in on of these passages so the cats can't get it). Don't forget to cover drain holes and put a ramp in your pond if you have one, they don't swim very well.
If you have a snail problem and a pond with tadpoles in the spring, chuck the snails in the pond, tadpoles love them.
Some of our hogs
www.brickfieldspark.org/data/hedgehog.htm
Mike