Stop drawing walls as part of background.
This eliminates the need to drop bkgnd cache on each level change Instead use QGraphicsPixmapItem with walls painted on its pixmap

Btw, if some of you use QGraphicsPixmapItem and don't care about mouse clicks & collision detection on it, consider using:

myItem->setShapeMode(QGraphicsPixmapItem::BoundingRectShape)

as this greatly improves perfomance when using setPixmap on this item