This demo shows how to get the invulnerability sphere in the mini-tunnel behind the "fence" in the "tower room" near the red door on level 16.

The mini-tunnel is actually part of a small network of tunnels. There is one other entrance, which is sealed with a grate. This other entrance is in a well-guarded section of the mine behind the blue door. Access is through the twisty little passage in the floor.

The switch to remove both the fence and the grate is inside the mini-tunnel network, so it must be tripped with a guided missile. It is a much easier proposition from the grated entrance, and that's what this demo shows.

The demo starts at the main entrance. I go through the blue door, down the twisty passage, and locate the grate. (Note: all 'bots have been dispatched. :-) One guided missile later, the grate and fence are no more. Normally, I would save this invulnerability for my later encounter with the boss, but, for effect, I finish off the demo by grabbing the sphere and heading to the red door.