What is Lebanon's Beaufort Castle, and why have Israeli forces captured it?

The Israeli army has taken control of Beaufort castle in Southern Lebanon, crossing over the Litani river, which they had been using as a de facto boundary. Israeli Prime Minister Benjamin Netanyahu called the operation a "dramatic shift" in the Israeli offensive in Lebanon. The Israeli army now has declared all land between the Litani and Zaharani River a conflict zone. Details by Caroline Baum.