This is a good introduction to layered security that doesn’t get too far into the weeds. Defense in Depth is a technique that the military (and corporations) use to buy time by slowing down the determined attacker. It involves the application of multiple, independent, redundant, security measures. Its applicability begins on larger networks, but is good information for the general awareness for all. It’s written by Doug Clifton of Invensis (Wonderware) and hosted on automation.com.