Heaven's Ladder is revealed to be an Orbital Mass Driver named "Gargantia." The mass driver is, with sufficient power, able to launch objects with sufficient kinetic energy to reach orbit, hence the name "Heaven's Ladder."
When the decision is made to fight with Ledo, Dr. Oldham informs Ridget that it's time to use the key. At this point, it is revealed that the key is the master for operating the Mass Driver. Although it's not powerful enough to fire payloads into space, it can easily fire projectiles from long ranges, making it an effective and powerful weapon. Striker's Temple and Pinion's Castle are both destroyed by it.