I can't get very far in this tunnel

Hint 1 of 5:

There is a locked gate blocking access to the rest of the tunnel.

Hint 2 of 5:

You don't have a key for this gate; and it won't succumb to your smelter,
either.

Hint 3 of 5:

This is one gate that you're not going to open yourself.

Hint 4 of 5:

If you've been to the Test Tunnel on the level above, you may have found a
window that overlooks this excavation area.

Hint 5 of 5:

There are Cepheids working in this area.