Ammo Room on Roof
What a gorgeous position. If you stay with your gun and they don't blow your grate, you've got it made - one engineer can defend the whole base. All you've got to watch for are spies and soldiers. The sentry goes over the edge of one of the ends of the open area, and the dispenser goes RIGHT next to the lift to the sniper area - serious whappage ensues. If a spy comes up, your dispenser will detect him. If a soldier or pyro is firing rockets, you chuck grenades at him and repair your gun. Mmmmm, nice.
Ramp Room
Similar to the ammo room, shove your sentry on the back ledge hanging off the edge and let it pick people off. Shove the dispenser in the small doorway to the flag room, ready to det if they get too close. Just float around the ledges with a few EMPs in your pocket and away you go.
Flag Room
So many places! A dispenser on one of the lifts and the gun at the top by the other lift is a good bet, or place the gun slightly behind the other lift on the floor. The flag "room" itself is bad - grenades or rockets just bounce off the walls in there. Two other top locations are on the bridge or just over the door to the touchdown area - both are hard to maintain, though.
Under Bridge in Flag Room
When building just hold down jump and you float to the surface to get air. Very cool. Just have it facing the entrance to the grate and pick them off - not good unless there is a good defence already or the grate is blown and thats their route for grabbing the flag.
