$  Changes in the battlefield, taking place during and after the Great War, forced changes to fortification  $
$systems.  Brick buildings employed earlier, places of concentration and defence for massed infantry and     $
$artillery units, were replaced by scattered, reinforced concrete bunkers.  Polish defensive plan assumed    $
$building of limited number of fortifications, deployed in strategically important locations.  Fortifications$
$in border industrial region of Silesia were built primarily.  Until September 1939 fortified lines were     $
$built at Mlawa and Rzegnowo, in Beskidy mountains and at Varta river. Some bunkers were also constructed    $
$in Pommerania corridor.  Although sometimes unfinished, they won some time to the Polish forces at the most  $
$dangerous directions of German advance.                                                                     $
