I understand that many cattle’s were build on locations which were easily defendable and offered a commanding view of the local terrain. But since cannons had made castles vulnerable and they had been in the age of gunpowder been replaced by star first and the like, surely WW2 era weaponry should have made short work of them. Yet places like Monte Cassino and Buda Castle proved pretty resilient and took a lot of effort to reduce, deposit massive air and artillery attack.

Why?