[Mirek Wawrzynski]

Hi
depends of period of Polish defence.

On the begining to smash the command (army+capital) and main centre of Poland resistance (1. IX - 8 IX(
During the seige of Warsaw in the last phase of the defence it was simple terror or bandids German bombs attack on civil's homes. hospitals, power plants, ect.. There are many information from Polish civilians, who were under these attacks. For example quite severe bandids attack on civil population were done on the right side of Warsaw - Praga.
The losses among civil Polupation after this bandids German attack were very severe. Probably when you go to Vikipedia there should be some information in English about casulty rate.

This terror/bandids attack had great influence on breaking the Polish resistsnce.
German had repeted this bandids "trick" over Belgrad in April 1941.

Regards
Mirek Wawrzysńki

PS
German loved to call all what should be conquered as a festung/fortress. In fact the warsaw was not so big and strong fortress as it was presneded in many war-tiem German publication.
Second this is easy explanation and escaping form the responsiblity of mordering civil. Frotess is military word. It is such a German "PR" to cover their bandids habits towards civils made during the WW II.