Vympel

If it was the mere footsteps of AT-ATs then it should be a steady rocking that doesn't stop, not to mention it should be getting worse as it gets closer. The impacts are intermittent. For that reason alone it doesn't work, and it still doesn't explain Han saying the command centre had been hit. The only reasonable explanation is that they're being struck by weapons fire. The only reasonable source of that weapons fire is AT-ATs. EDIT: the novel: As for seeing the effects of what hit the base - we never directly saw the base get hit in the first place. So there's no contradictory visuals. Note this does not mean I think they were firing KT beams at the base or whatever, I'm just pointing out an instance that clearly points to pretty powerful weapons generally speaking.

No, Echo Base was clearly under a rock mountain/hill whatever you want to call it. We can see the rock that makes up the mountain in external shots of the base. AT-AT's must have powerful cannons to cause damage to the interior of that base - as is clearly implied by the film with the base being rocked by explosions, and character testimony: "I heard the command centre had been hit!".