| Bill Lumberg |
The Martian war machines should be fully insulated from electical attacks and non-rusting. Consider making them a type of metal that does not exist on the PCs' world or be a non-metallic material that looks like metal.
Make the walkers superbly balanced. IF the PCs turn the ground to mud under them make it little more than a nuisance. Vary the heat-ray to encompass other types of energy. This will challenge fire-resistant PCs.
I considered this once as the basis for a campaign. The world would have been conquered by the Martians and the PCs would be descendents of the survivors. They would come out from the ruble to fight the Martians, free humans from "cattle pens", fight traitorous humans who had sided with the Martians etc...
I decided it would be a low-magic world to make the Martian more challenging.
I would also suggest making the Martian technology psionic in nature. This makes it unlikley that the PCs can make use of it if they capture any of it.