Basically it goes like this:
1) Let the Alliance cap Horde Keep
2) Grab your siege engine
3) Go back to your keep
4) Don't Recap it
5) LolSiegeFarming
6) They will run out of reinforcements in about 5 minutes (They will keep spawning in the horde keep GY)
Get someone competent on the gun, and driving the siege and it's a guaranteed win. Just keep the siege protected from glaives.
I assume this could be flipped for the alliance too.