Technique · rendering
Background objects
Large enemies or machinery drawn in the background's tiles instead of with sprites, so they can exceed the sprite limits, at the cost of moving only by redrawing or scrolling.
Sprites are few and small, so a game can draw a big enemy, a door, or a bridge into the background instead. The object then changes by redrawing tiles, and it can move only as far as redrawing or scrolling the background allows.