ray_afraid said:

JediTray said:

From what I understand, the fall knocked out the AT-AT's shield generator, allowing it to be destroyed by the Snowspeeders. Also, when someone refers to "armor", it doesn't always mean just the physical structure. It can also refer to any protective shielding, since it is in effect part of the vehicle's defenses.

Make sense?

Nice try, but the AtAt's have no shield generators. And If they did, how was Luke able to climb up and cut through the bottom hatch?

For the same reason the Empire had to penetrate the rebel planetary shield with the at-ats in the first place instead of just bombing the base from orbit : a shield generator is meant to stop laser blasts and the like from reaching their target but they can't stop solid moving things, including humans, to go through them.

However, the "official" answer to your original question is that despite its heavy armor an AT-AT has one very weak spot : the flexible and unarmored neck. Which happens to be what the snowspeeders hit right before the walker explodes.