Sometimes the procedurally generated levels will procedurally generate junk that blocks your view of the incoming hordes. Before starting the assault stage, clear your sightlines, taking down trees, boulders, and whatever else is blocking your vision. Most husks spawn far away and pistols are extremely accurate, making it easy to pick off enemies before they even get close. At the very least, it's helpful to chip away at their health and let traps finish them off if you're too busy trying to figure out how to make a staircase.