Inspiration

[faraway]

... That card board is about 1 mm thick ....

The cereal boxes, at least in Germany, are about 1/2 mm (0,02") thick. That is not thick enough for the buildings of that size. I bought 1 mm (0,04") and 1/2 mm (0,02") card stock when I started with the project. The 1/2 mm card stock is still unused as it is not stable enough.
It is no problem to get a stable structure out of thin card stock, even paper can be used if the construction is modified. The problem are the walls. You need at least 1 mm (0,04") to get very flat walls. They are large objects and look lousy if the thin material bends between the fix points.
Just my two cent

I went to the balcony to take pictures without the flash to show more details