I'm not sure if this way to get in was ever posted but here I go...
1. Die in IF somewhere.
2. Face the bank from the AH bridge and there will be a tunnel to the left, go into it.
3. The wall straight ahead when you go in the tunnel will have torches on it. The first 2 torches along that part of the wall will have a ledge beneath them jump onto it.
4. Run against the wall left and right and if you notice your guy will begin to "Sink" into the wall.
REMEMBER: YOU MUST BE DEAD