my sentry aways get destroyed by spy of splash damage from soldier rockets. If you walk up the the fence with a gun you cant shoot because you are too close. If you back up just a bit you cant hit the sentry...the only thing you cant hit is the teleporter that is deeper back in the cave or the dispensor....that is what i would call an exploit. and I never leave my entry teleporter working to trap other players.
I guess that I will stop doing that. the sentry gun dosent get points there as other places. Just remember that you can kill the sentry gun with spy,soldier or any other splash damage..aka demoman. pyro but not too close to the fence. or just any gun.