Guvnor,
Locked doors.
According to reports, the passengers on the flight that went down in Pennsylvania became aware of the intent of the terrorists, and decided to risk their own certain death against the possibility of further "collateral deaths" , and took some kind of action. The aircraft went down in an unpopulated area, and did not cause the considerable mayhem it might have. If these reports are true, there were some truly brave people on that aircraft.
My point in an earlier post was that now the Capt may have to determine whether to risk certain death for many against probable deaths of some. Not an easy decision, but "the lesser of the two evils" might well be thew choice.
Firing position for the crew.
With a locked door, there would be some notice before the terrorist managed to force his way in. Presumably when he finally broke in he would be looking straight down the barrel of at least one weapon.
Skymarshals
I think there have been several incidents with skymarshals where there was an acceptable outcome, but I do not have access to some of the classified info.
It is notable that El Al do not have much trouble with inflight interference of any type. They must be doing something right.