A fun little video by Bofner talking about how two Castlevania games on the original Nintendo Game Boy achieved a certain visual effect that wasn’t technically possible:

What I really liked was this sequence of events:

• Game Boy introduces a special immovable layer to accommodate HUD/GUI elements within games (such as score displays or life meters),
• Castlevania programmers come up with a creative hack to use the HUD layer draw a movable spike wall instead,
• then, they use another hack to draw the HUD anyway, outside of the HUD layer.