15 feet in a rural area somewhere. Inside it have a bathtub, a wood burning stove, a kids play area, plenty of Meals Ready To Eat, an abundance of water, some of your favorite DVD's, a full scale kitchen, a shower, a 500 gallons septic tank, a few different types of generators, an air filtration system, a nice deep well. And of course as you are shopping for the end of the world, you want to make sure that this form of steel cylinder has some solid steel blast doors, super strong walls that can withstand mortar shells and electromagnetic pulse. It must also protect you from winds up to 500 miles per hour, 10.5 earthquakes, and if a flood comes then this cylinder must be able to submerge and float for days. And not to forget about Meteorites and heavy falling debris that weigh at least 1000 Lbs. And not to forget about personal protection built-in within such castle steel cylinder, from those who would want to harm or even kill you and yours before your time.