All obstacles and movements are identical to Classic mode, but in Cave, the game screen is shaded on the edges, to simulate moving through a cave, where the center (position where the ball is located) is source of light. This makes the gameplay more challenging, as the player's field of view is smaller, therefore sometimes it's hard to see incoming obstacles.
In early levels of the mode, the field of light is relatively large, but as the game progress, it gets smaller. In certain levels, there is a dynamic light field which shrinks and grows. Sometimes, it shrinks in the way that the ball becomes the only visible object on the screen (for about one second). This is considered the hardest part of Cave.