Defending Minas Tirith a strategic error?

[Boromir88]

Actually sending Faramir to Osgiliath was Denethor's only mistake in defending Minas Tirith. Denethor learned a lot from the palantir, he knew where Sauron was going to attack, and where he was going to hit hardest, he did a very good job defending Minas Tirith, for the fact I mean not one enemy entered into the City. For it wasn't Gandalf who set up the defenses, Gandalf was sort of that motivational pep talker, Denethor was the man who set up the defenses. When Gandalf got to Minas Tirith, the beacons were lit, Hirgon was sent out, walls were set up outside of Minas Tirith, so Denethor did a very nice job defending the city. Too bad the palantir made him crazy in the process. The gates broke and Rohan arrived. I do agree though if it wasn't for Rohan Minas Tirith would have fallen. But if it wasn't for Aragorn, and the 50 ships full of men from Lebennin, the rohirrim would have gone down with Minas Tirith.