Seriously, I thought it was pretty awesome too. I've conceptualized something similar and man would it be cool to realize.
Have you heard of underground shipping container bunkers? Shipping containers are fairly easy to find and all you have to do is bury them with a few feet of dirt on top and then cut doorways into them. Not a bad gig and you don't have to worry about the minimal space inside of the cylinder shaped bunker.
but neither is burying the other cylindrical bunker. Burying anything involves a lot of work (I have experience with backhoes and large equipment)... But I mean come on. An underground bunker full of beautiful tools of death. Lets be honest. Who isn't willing to play in the dirt a bit?
it pretty much all comes down to how much you are willing to spend in the end. Which do you think is cheaper? In my opinion "pre-made" cylinders might cost more than shipping containers but then again buying the material to support such a structure might even them out or end up being more after its all said and done. I think one of the pluses of a square shape is how "stackable" they are. Better usage of space.