A big concrete bunker would definitely be better, but it takes a lot more energy to build (forgot the exact number, but it was 10 times or more), and I just couldn't bring myself to do that. We originally wanted to buy an old place and renovate it, but just couldn't find the right spot after 3-4 years of looking, and so we opted for the eco-passive-blahblah-house (with a lot of DIY).
BTW, if all goes well, I will probably start a second blog to describe what we've done. There's some funny/interesting stuff there, but we have to wait and see how things work out.