Learn GIMP from the master.
I'm going to start with these pictures:
spoiler: these pictures
We'll start by taking some walls and correcting the perspective with the unified transform tool. Remember to save.
We can now paste pieces of our perspective-corrected wall to the towers. Remember to save.
After merging some layers I add a mask so I can erase and uncover the layer as I please, by painting black or white. Remember to save.
I adjust the lightness and color temperature of the sun-facing walls. Remember to save.
Finally, using the same tools as before, I add the sign. Remember to save.
now go make me proud
Remember to save.