What made Krak Des Chevaliers have such a strong defense?

Answer 1

It was built on a hilltop. This forced the attacker to have to climb the under arrow fire from the defenders before reaching the walls. the rugged ground made it very hard for siege weapons such as catapults, siege towers and battering rams to get in effective range of the walls or reach them effectively. The hilltop position also allowed the defenders to survey the surrounding ground and observe invading the invaders, troop movements.

it was a concentric castle: it had a low wall on the outside and a higher wall on the inside. This allowed the defender on the inner wall to fire projectiles over the outer wall, while the outer wall defenders fired as well, concentrating their firepower.

If the outer wall was captured, defenders could retreat to the inner wall and continue fighting.

The castle entrance led to a corridor with holes in the ceiling (murder holes). The attackers had to pass through the corridor it to enter the castle, while defenders threw boiling water and project lies through the murder holes

The castle was large enough to hold a few thousand soldiers. An aqueduct supplied them with water, which meant they could survive long sieges without running low.