Why is a gateway the only way to get out of the Black Tower? Yes its a walled enclave with only a few enterances/exits that are guarded but what is to stop a few channelers who are strong in earth to tunnel under the walls and simply walk/crawl out of the compound.

We know that men find it a lot harder that women to sense other men channeling. Even if they do detect channeling its the Black Tower. Men are encouraged to channel as much as possible so how can anyone pick up the difference between the tunneling and any other number of uses of the one power as long as the channeler is not stupid enough to do it right in front of a whole load of Taim's cronies.