Roark represents himself at the trial, as usual. He should just take the bar exam at this point.
He chooses an unsympathetic jury, deliberately. Because otherwise the trial would be boring.
Keating delivers testimony like an automaton, or a robot. He's been totally defeated by Toohey.
The basic case against Roark is that he illegally blew up a government housing project meant for helpless poor people.
Roark makes a speech about his beliefs, though, and gets everyone pumped up about individualism, artistic integrity, the coolness of architecture, capitalism, truth, justice, and the American way.
The jury acquits him for his awesome speaking skills and just ignores the fact that blowing up buildings is totally illegal.