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This is my first attempt, also done in GIMP. I managed to somewhat translate the layer styles in part 2 such that they worked in GIMP I think.

For the buildings I duplicated the building layer and then added a bevel to it (Filters->Decor->Add Bevel...). I then used the emboss filter (Filters->Distort->Emboss) to add an embossing effect. I then put this layer above the buildings layer and set it's mode to Overlay. I did a similar thing with the walls too.