One of the most important things in security is to setup a practical set of security features. A layered approach is to me one of the best approaches and it allows you to protect the most important things and make sure they are hard to get to. Lets take a quick look as it applies to home security.
In the case of home security, you buy a house in a nice area. First layer. Then you light it up. We have street lights in front and one in the back (for about 10 bucks a month). There are no dark entrys around my house. Second layer. Then we have a good alarm system. You can hear it for 2 miles around. Third layer. You do not want to find out about my fourth layer. I will be very painful to whoever cracks the other layers.
Software development security is sort of like home security. Over the next few months, we will explore lots of areas that are tied into securing applications.