The shield generator was strong enough to withstand orbital bombardment from the Imperial Fleet, but it did not protect the entire planet of Hoth. AT-ATs were able to land far away from the Rebel base and approach from the ground. The AT-ATs were tasked with destroying the shield generator so that the Imperial fleet could commence orbital bombardment:
General Veers: My Lord, the fleet has moved out of lightspeed. Com Scan has detected an energy field protecting an area of the sixth planet of the Hoth System. The field is strong enough to deflect any bombardment.
Darth Vader: The Rebels are alerted to our presence. Admiral Ozzel came out of light speed too close to the system.
General Veers: He felt surprise was wiser...
Darth Vader: He is as clumsy as he is stupid. General, prepare your troops for a surface attack.
AT-ATs were the safest way to attack the shield generator. Their armor was too thick to be penetrated by the Rebels' snowspeeders:
Luke Skywalker: That armor's too strong for blasters...Rogue Group, use your harpoons and tow cables. Go for the legs. It might be our only chance of stopping them.
The plan was that the AT-ATs would only have to destroy the shield generator, and this would allow orbital bombardment from the complete safety of space. Other ground forces would have been unnecessary. Meanwhile, the Imperial fleet would prevent Rebel forces from escaping the Hoth system since the Empire's spaceborne forces were vastly superior to the Rebels'. However, the Rebels were able to use their ion cannon -- which the Empire was unaware of -- to disable the Star Destroyers long enough for Rebel transports to escape:
Princess 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.
Derek 'Hobbie' Klivian: Two fighters against a Star Destroyer?
Princess Leia: The ion cannon will fire several shots to make sure any enemy ships will be out of your flight path. When you've gotten past the energy shield, proceed directly to the rendezvous point. Understood? Good luck.
It was a very "safe" plan that should have resulted in minimal Imperial losses, but the ion cannon made the Imperial fleet less effective and the use of the tow cables resulted in unexpected AT-AT losses (though the AT-ATs did succeed in completing their objective).