There's an open letter from the Death Star's designer Dak Exhaustport explaining why it wasn't his fault the Death Star was destroyed by a group of small fighters. While I do understand his concern regarding the 90° turn at the vent entrance and I too find it suspicious. I have to challenge his claim that he is without fault. It is true that there was a conspiracy to destroy the empire. It was clearly being carried out by Anakin Skywalker and his son Luke. Clearly it was meant to exploit the emperor's desire to bring Luke to the dark side and it was meant to take place when they met and not sooner. Unfortunately the whole Death Star thing blew up in Darth Vader's face and he had to postpone the meeting between Luke and the emperor.
Now back to the Death Star vent port issue. Dak Exhaustport claims it isn't his fault. That other issues lead to the exploit of the vent and not his lack of foresight. I say that it is not only a design which exposes the battlestation's core to a fighter attack, it is also a very inefficient one. Even for Earth's standards nowadays. Let me present exhibit A. It clearly shows the vent port going from the surface straight to the core. Literally straight to the core.
So while I do admit it is possible to use the force to turn the torpedoes 90° at the entrance of the vent, it would have been impossible to guide them through a maze of turns all the way to the core. The force is strong with Vader and Luke, but without visual sight it would be impossible to guide regardless of their skill with the force. This leads me to exhibit B, C and D.
Exhibits B and C show modern day Earth ventilation designs in which turns are clearly visible. Hot fumes can easily handle turns in the ventilation system. If Dak Exhausport had placed a few turns in the shaft as it lead down (or up, however you see it), the Death Star would still stand.
The design was also a poor one as Exhibit D shows how one shaft usually terminates. It fans out to a set of actual ports that provide even ventilation to the area the vent is meant to cool. So we would expect the controversial vent to fan out as it approached the core. This makes sense because a single port would be able to cool the area in its vicinity, but not areas in the opposing side. This would lead to thermal differences within the core. If the vent fans out into a set of vents around the core the overall effectiveness of the vent system increases and removes the possibility of a torpedo reaching the core.
So though there is the aggravating circumstance of the whole Skywalker conspiracy, the fact stands that Dak Exhaustport is entirely responsible for the destruction of the Death Star. Not only is it a poor vent shaft design as shown by exhibits B to D. It is also on that leaves the whole battlestation susceptible to external attack by a lone fighter pilot.