I haven't tested this for horde but I think it must work for them if their gate opens a similar way as the Alliance one. When the match starts go stand under the gate until it closes facing the east tower on the gate like this:
Then wait for the gate to close (it won't work if you try while the gate is still open) and just walk forward until you walk into the tower on the east side of the gate.
When you're in there it's a great spot for disarming bombs. I got my achievement in a couple IoC matches.
You can walk out of the tower to get closer to bombs that people put out of reach and still be able to get back inside the tower as long as you stay next to the gate.
I know I'm not great at explaining but I hope the pictures speak for themselves.