(This question is related to How did the AT-ATs get to the surface of Hoth?, and was originally posted as an answer there, but it's not the same)
In The Empire Strikes Back, the Empire attacks Rebel Echo Base located in Hoth. The Rebels are alerted before the attack, and raise some sort of shield to prevent orbital bombardment of their base. The Empire is then forced to deploy land forces to first disable the shield generator, before destroying the base. Presumably, the Imperial forces are deployed using dropships.
My question is about how the Rebel shield generator works, and how the Imperials manage to bypass it.
I think the shield generator is something of a plot hole. At first I incorrectly thought it was a planetary shield (i.e. a shield around all of Hoth) preventing orbital bombardment, and also preventing any dropships (with their AT-AT payload) from landing, but I think Wookiepedia got it wrong: the script states it is an energy field around just an area of Hoth, presumably Echo Base:
VEERS: My lord, the fleet has moved out of light-speed. Com-Scan has detected an energy field protecting an area around the sixth planet of the Hoth system. The field is strong enough to deflect any bombardment.
So my initial objection ("how did the Imperial dropships bypass the shield?") has been answered: they didn't. They landed outside the energy field to deliver the AT-ATs. But now I have another question: does an AT-AT have to walk inside the field to fire at the generator, or can it fire into the field from outside?
Maybe the field simply cannot stop matter, only energy weapons (therefore preventing orbital bombardment), in which case the AT-ATs would be able to just walk inside. However, the script also states:
LEIA: All troop carriers will assemble at the north entrance. The heavy transport ships will leave as soon as they're loaded. Only two fighter escorts per ship. The energy shield can only be opened for a short time, so you'll have to stay very close to your transports.
So it's clear that the energy field is also capable of stopping matter, in this case troop transports and fighters. It follows that it should also prevent AT-ATs from walking through it. So the only way I can see the walkers disabling the generator is by firing into the energy field!
This would mean the field is strong enough to stop matter such as ships and fighters, and energy weapons used for orbital bombardment (and also the Ion Cannon the Rebels use as an anti-orbit battery: Leia also states they must lower the shield to use it), but not able to stop whatever energy weapons the AT-ATs use!
So how on Earth did Imperial Walkers manage to disable the shield generator?