I ended up needing to replace my metallics to finish off the tower. The old 'bolter shell' pots of GW paint are abysmal for not drying out. Once I was reequiped I did the metal work around the wheel of magic in gold, with a brown wash, and picked out the symbols themselves with a silver.
The clockwork I was/am a little torn on. I do like it in a copper, but there isn't really enough going on in there. I think I was the black background roughly coloured (drybrushed perhaps?) in copper as well in an internet pic, and think I might like that. But I can imagine if I don't do it right it will look awful and detract from the gears themselves. At least it is mostly hidden in shadow when the tower is assembled.
I did like the bricked up archway with the hidden symbol. For those in the know this is obviously a sigil to tzeentch, the changer of ways. This chaos god is closely aligned with magic, and I like to think that this helps explain what has gone wrong in the tower and resulted in it's partial destruction.
Afterwards I tried to add some fungal/lichen growth where I thought it would be (I live in a damp climate, so it makes sense to me that there will be green crap growing in cracks everywhere it's wet.
The obvious one was by the drainage hole at the base. I also did a bit in a few other parts of the base. The few remaining wood singles got a dose as well.
I remember seeing old 'eavy metal pics from white dwarf that had a fuzzy fungus thing used. I had a hell of a time finding any information on how to do it though. The closest I got was in the hobby store the woman said that painters used to use pastels and shave them with blades and mix them with glue. I ended up using a vallejo earth tone/pigment mixed with some ink and white glue. It gives a bit of texture...not as much as I had hoped though. I may just need to add more and build up layers though.
Here is the final (ish) product. I doubt I'll get around to doing much more besides sealing it at some point in varnish. Perhaps when I bring it out on the table comments from the gamers might inspire (or shame!) me into doing some more though.