Hence my post about not just anyone being able to bash open the gate. 
Its not a door. It should be possible but require something...special. Not just some grunt caliban with a Bull's potion and a crowbar.
Siege equipments and tools. As Soren said, it's not a simple lock. It has a mechanism activated from within the gate house. It must be strong enough that no Hulking humanoid is able to break it alone.
