I'm finally going to have a worm bin that is EASY to use. I am not sure I agree with this website when it says that castings will drop down, though. I think you have to rotate the layers, harvesting the bottom each time.
Earthworms are amazing garden pals – and powerful composters. Learn how to harness their talents by vermicomposting, and start your own home DIY worm farm! http://gardenerspath.com/how-to/composting/worm-farming-vermiculture/