However, you have the "gates of doom" between T5 and T6 already. I don't see how this is any different between T4 and T5. If you want to buy yourself more time have gates of doom on either side of the tunnel so that the far gate can be shut once sterility is breached.
On a side note, I hate the "I know why they were but I can't tell you" type responses. If you can't answer the question, then don't.