Natural sponges are fascinating structures.  A sponge is irregularly shaped and has no clear entry or exit.  The very idea of a “perimeter” seems completely ill-matched to a sponge.  Once water gets into a sponge, the water can travel to any other part of the sponge through what seems to be a practically infinite number of permutated paths.  And so, your network is like a sponge.
A modern, sophisticated network has no real perimeter.  People connect wirelessly, they bring in USB drives, they take home their laptops, put disks in the CD drives, and so on.  Never mind the ever-present threats of spam, spear phishing, and accidentally downloaded trojans.  The bottom line — bad things will always find ways into your network.
Once something bad has made it into your network, what next?  Like water, the bad stuff starts sloshing around, moving through and infiltrating all the dark nooks and crannies of your network.  All those servers, processes, paths, and links that the IT staff forgot about long ago will be found by bad things.  Remember those links configured by the system administrator who left two years ago?  You know, those router settings everyone needed right away because of the IT emergency.  It turns out that nobody remembers those configs.  Guess what — because your network is like a sponge and the bad things slosh around like water, some malicious actor WILL find those forgotten configs.
Change the way you think about security.  What you need to do is recognize that there is no perimeter, and there are a myriad of forgotten pathways running all through your network.  Instead of thinking about setting up security gates, think about how you can actively track down intruders.  Traditionally, you need very experienced staff working full-time to track down the bad stuff in the network, and then take appropriate action.  The scope of modern attacks — the volume, velocity and variety — demand automated defenses.  And that’s why we make the Ridgeback.  We want to turn that sponge topology inside out and harden the entire network, rather than keep it porous as it is today.