Olgoy-Horhoy

Hello everyone! I'm building my little safe fortress but can't understand how to make second level. I belive it's possible because I saw that on screenshoots (for example http://cdn4.steampowered.com/v/gfx/apps/108600/ss_52a9cea24c9d205a2d3462ca9ccba6d8bdf61e6f.1920x1080.jpg?t=1384539150). I couldn't find any information on wiki or this forum so, can anyone help? After I make the stairs, how can I force my character to build above ground floor? Thank you!