Thats part of what has already been posted here. The way the buildings were engineered and built allowed them to take horrendous punishment and remain standing. The design wasn't on purpose as the city was built years before the battle ever occured even before the war started, the design was for ease of construction by highly unskilled labor. The communist liked to keep idle hands busy, that way they didn't have time to plot their own new governments.
The Germans tried everything they could to pulverize the city. If you read the engineering text about the seige you will find that most archetects can't believe that much ordinance was used and that much of the city still intact. Matter of fact, you can go to the city now and in the older parts still see where they just rebricked the holes and were done.
Concrete and Steel compliment each other tremendously. They even have some original areas still standing. After over 60 years and riddled with holes, these structures still stand. Entire floors blown out of them, yet they still stand supported by the exterior walls.
Their own construction allowed them to hold on to constant cover and concealment.