The Fortress of Solitude is Superman's Kryptonian base. The first Fortress was a huge iceberg. A huge golden key that only Superman or Supergirl could lift was needed to open the door. Superman built it. The second Fortress was a huge crystalline building. It looked similar to a pile of crystals and icebergs. A green crystal built the second Fortress. The third version was similar to the second one, but was bigger and looked different in the inside. This version was created by the Crystal of Knowledge, a ring like crystal.