My guess would be that the buildings that were completely leveled were older construction (brick/wood) and those that remained as shells were newer (concrete). Those around the church would logically have been old wood/brick buildings. Any older walls that did survive were probably well coursed (block) stone.
Wood building burn down easily, entire brick buildings could scatter with one bomb, while concrete typically loose roofs and floors and only partial walls were direct hits took place. Case in point, in Stalingrad, much of the debris from concrete buildings stayed inside the outer walls, while brick buildings spread themselves out into the streets. The smaller the room a bomb explodes inside, the more damage is imparted to the walls. Brick building usually have small rooms because they cannot support extremely long roof/floor beams, wood buildings even more so (smaller yet).
There was a thread else where that analyzed the effects of bombings on different types of structures in citys. The church, being made of heavier stone would absorb more of the blast, and its large open spaces allow the force of the shock waves to disperse rather than imparting destruction (similar to concrete buildings). I would imagine most of the church's windows were blown out regardless, either from shock waves or flying debris.
BTW, looking great. The task is a daunting one, looking forward to the beta (even if takes a year)
