Fortitude is an ICBM interception missile silo. Built by all T2 land/air cons it is able to cover a decent area spanning over your base and surroundings. It protects you from long-range nuclear missiles (not tactical nukes). Takes 90 seconds to stack up one missile. 

Always make one after the 15 minute mark and be careful as it may be EMPed by spies or Armada EMP silos.