I have finished off the first hill, built the barricades, and made a start on the second hill. I have dropped the idea of a metal planet; I couldn't make it look right, and fell back on the 'red martian soil' concept. And it gives a contrast with the dark metal, which is also good.
Building the second hill is easier, since I now know that I don't need holes in the base board for the pegs; the lowest cog is tall enough to make a sturdy foundation for the platforms. I simply pushed all the components together, checked for alignment and height with a couple of figures, and
then epoxied the bottom of each cog to the board. The axles slip in easily enough, so I will paint the parts separately, clip the axles to length and assemble afterwards.
Next step will be to rifle the local charity shops for a set of plastic Meccano to make some buildings from...