Why did Zeus let Troy fall?

Why did the supreme god, Zeus, ultimately allow the city of Troy to fall, despite the numerous heroes and warriors who fought valiantly to defend it? Was it a test of human resolve and perseverance, or was there a deeper reason behind his decision? Did he have a hand in orchestrating the Trojan War from the beginning, or did he simply sit back and watch as events unfolded? And what does this decision say about Zeus's character and the nature of the gods he rules over?