How do you build a Underground fortress the best?

Question

I am building a large underground fortress but i don't know the best way to make it, or the right amount of measurements so it doesn't cave in. I want it to be at least 3 floors deep.

Answer 1

I wouldn't know but it sound very interesting--what are you building it for??

Answer 2

Drill tunnels into a hugh mountain and reinforce the ceiling and walls with steel reinforced concrete.

Answer 3

You should talk the Viet Cong that built the tunnels of Cu Chi, the Austrian bunker dude that kept hot chicks n stuff underground, and the Taliban molemen that constructed Tora Bora.  In addition to that, you should consult with a Soil Engineer, Civil Engineer, and a good Geologist to obtain construction advise specific to your job site.