What a great build!
Can you explain a bit more about how you went from the first WIP picture (a basic box) to the second one (with windows, battlements and corner detailing) as that step seems like magic to me!
I cut most parts with a Proxxon table styrofoam cutter. Look up Youtube videos to see them in action. It is an indispensable tool.
The corner pieces started out as a long flat piece. I cut the alernating jagged edges that would result in the cornerstones and then sliced the piece into six thin strips. Those were then scored, folded and glued over the box's edges.
The windows were made in the same way. First I cut a block into the window shape, including the window opening. Then I sliced the block into thin bits each thin enough to be glued onto the outer wall of the box.
The battlements started out as long thick flat ends. I first cut one side into a 90 degree point (making the top) and then cut into the top at regular intervals to make the battlements. Finally, the pieces were cut to size and made to lock into each other at 90 degrees angles. Then the whole battlement was glued on top the the box.
Then I engraved all the masonry into the styrofoam with a pencil (the graphite provides a nice lubrication preventing tears) and separated the various floors from each other with a hot wire hacksaw. I deliberately did this in a random jigsaw pattern so that the lines would vanish into the masonry and the pieces would fit snug into each other.
Then I added the floors and so on.