[dragonwriter]

No, Bastion forts are not convex polygons like the Pentagon, they are concave because the concavities create kill zones where the fort can fire at attackers from the sides as well as the front.

The Pentagon is an (irregular) pentagon because of the roadways surrounding the site when it was selected.

[bryanlarsen]

The original chosen site was on Arlington Farms and was pentagonal. The current site wasn't, but they (mostly) kept the original design.

[saagarjha]

Is the Pentagon not regular? It seems like it is from pictures…