how do the empire get the at-ats onto hoth?

This would be the sensible answer. Presumably there are some sort of cargo transport ships that fly troops and ground combat machinery from the star destroyers in orbit, to the surface of some unlucky planet. I presume that a working Death Star had the capability to carry enough of those things to conquer an entire planet at surface level, without getting into the planetary gravitational field.

Luckily, they're made by ikea, so assembly isn't too difficult if you can read pictures.

lmao!