I've already mentioned that I undercoated the fortress in plack. The idea here was that should the painting miss any crevices , they would look shaded. As it turns out, I don't think i missed too many bits!
The model was painted in Tamiya spraypaints. Base coat was the palest gray I could find with a few oversprays of a slightly darler gray. I then painted back with the lighter gray to blend the darker in in places; the idea was just to vary the base colour a little. Spray-paints are great in terms of their very high quality finish, but so need to be broken up as this finish can look quite artificial.
The next layer of colour was some smallish, randomly-applied patches of a light tan which I applied to about 30% of the models' vertical surfaces, not being too bothered is some went onto the horizontal surfaces, but not aiming for it either. Finally I sprayed areas of green patchily onto the bottom fo the vertical surfaces, allowing it to drift 'up' a little to simulate algae and other water-borne nastiness that you might get if the fortress was standing in a wet moat.
The last touch was to rub some chalk pastel onto my finger-tips and then roughly over all the raised areas of the model. Some red, orange and yellow ochre. Less is more here and the idea should be to apply tiny patches of pure colour to slightly brighten what to now has been a fairly dull colour palette.
The chalk was sealed in place with a spray fixitave which can be got from any art store. This needs to be done because dry chalk pastel rubs off very easily.