http://i.imgur.com/aaP80dH.jpg
So i spent some time during the past 4 weeks, drawing and playing architect every now and then. I did some proper calculations on various designs, and this turned out to be the absolute most efficient in terms of wood/metal spent per unit of safety. By unit of safety, i mean per C4 that HAS to be used.
Some explanation: The design is based on a 100% "Grief-Proof" system, where the only way to the top floor (Where the loot is) is from the ground up (Of course you can blow yourself in through the 2nd floor walls, etc)
The outermost foundations that go all the way around, are nothing but foundations with a midpillar reaching all the way up, and 1-2 stories ABOVE the top floor.
The 2nd outermost foundations that go all the way around, are foundations with edgepillars and midpillars all the way to the highest floor, where you will place ceilings ontop, in order to create a "ceiling-cap" The ceiling cap with midpillars underneath, combined with the outermost midpillars all the way up and beyond the tallest floor, creates a system where no one can build stairs or jump ontop of the roof, and jump onto pillars in order to reach any floor above the 2nd floor.
This design essentially means that someone could blow into the 2nd floor, where the stair to the 3rd floor is located, and thus bypass the 1st and 2nd floor entirely. From the 3rd floor, and all the way up, to as many floors as you'd like to add, one HAS to go through 4 metal doors. This means that 8 floors = 20 Metal Doors minimum, 9 = 24, 10 = 28 and so on.
The numbers and calculations on the picture are based on an 8 floor building.
Feel free to discuss!