Entrance Ledge
Some Engineers place their gun on the ledge just above the door, but a few MIRV grenades will soon take care of it. Having it all the way up will get most attackers. Supplies are very close at hand, and having the dispenser before the door and then blowing it whenever the door opens is an effective slam in the face for most would-be gassers - just keep 100 cells close by to rebuild it...
Yard Rails
...It's always sad when an engineer hits the rails... No, seriously. Place your sentry on the rails - but not in the center, that makes it an easy target. The enemy will not expect it to be offset to one side. With the dispenser blocking the way from the showers you should be able to avoid most spy attacks. Be careful of scouts CJing up to the rails, your gun might miss them. Make sure the gun is opposite the doorway to the Wardens Office, as it gives it more chance to lock on to those key stealers...
Wardens Office
"Woah! Death By Stereo!" - The Lost Boys. A truer word was never said. If you place a gun on one of the speakers, you'll TRASH most of the enemy as they run in - they won't realise where the gun is, and grenades will do minimal damage because it's slightly raised. The dispenser makes a good first line of attack here as well - just det it as soon as they run past you - 'cos you'll be outside baiting them, remember?
Gas Room
If they have nippy scouts, or if there are multiple engineers, it's a good idea to build next to the gas room. Any enemies who run in will instinctively back off and attack the Dispenser rather than risk being blown up by it - at which point the Sentry will chew them up like rag dolls. Bwah ha ha!
