"Walls are for the weak and untrustworthy" - u/Jackeea
The main idea behind this design is to make it minimal and as compact as possible. Compactness causes attacking biters to get in range of not only the turrets in wall facing the assault party but also the turrets in far wall. So even though total count of turrets is low, the effective firepower is, well... effective. Due to the compactness the repair bots can respond to damage very rapidly, even if their speed is not heavily upgraded. The outposts round-ish shape is also intentional - exposed corner turrets tend to draw too much spitter attention.
Image of earlier, less compact version fighting biter onslaught on a deathworld.
